feat(workspace): turn a branch into its own note, merge notes

- Extract: button on the bullet (desktop) or /extract (/notiz). The bullet
  becomes the title (hashtags stay at the link), its children the content
  of a new note in the same folder with the source's shares (a foreign
  source owner gets write access); the branch becomes [[title]] through
  the operation log, so open editors follow.
- Merge: drop a note onto another in the sidebar or use "Mit anderer
  Notiz zusammenführen…" in the note header. The own source is appended
  as an h2 section with its bullets below and goes to the trash; the toast
  offers a complete undo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/WorkspaceView.tsx

@@ -27,6 +27,7 @@ import {
 import {
   Document,
   DocumentBlock,
+  MergeResult,
   DocShare,
   SharedFolder,
   TagDetails,
@@ -47,6 +48,7 @@ import { sendDocOps } from '../../lib/docSync/oneoff';
 import { useMobileActions } from '../../context/MobileActionContext';
 import { DocumentEditor, DocSaveStatus } from './DocumentEditor';
 import { TrashView } from './TrashView';
+import { MergeDialog } from './MergeDialog';
 import { CountBadge } from '../../components/CountBadge';
 import { extractHashtags } from '../../lib/hashtags';
 
@@ -87,6 +89,9 @@ interface FolderTreeItemProps {
   handleShareFolderClick: (folderPath: string, existingSf?: SharedFolder) => void;
   handleDeleteFolder: (folderPath: string) => void;
   onSelectDoc: (id: string) => void;
+  // Dropping a note onto another one offers to merge them
+  docDropProps: (targetId: string) => React.HTMLAttributes<HTMLDivElement>;
+  mergeHoverId: string | null;
 }
 
 const FolderTreeItem: React.FC<FolderTreeItemProps> = ({
@@ -106,6 +111,8 @@ const FolderTreeItem: React.FC<FolderTreeItemProps> = ({
   handleShareFolderClick,
   handleDeleteFolder,
   onSelectDoc,
+  docDropProps,
+  mergeHoverId,
 }) => {
   const hasContent = node.subfolders.length > 0 || node.docs.length > 0;
   const isExpanded = Boolean(hasContent && expandedFolders[node.fullPath]);
@@ -234,6 +241,8 @@ const FolderTreeItem: React.FC<FolderTreeItemProps> = ({
               handleShareFolderClick={handleShareFolderClick}
               handleDeleteFolder={handleDeleteFolder}
               onSelectDoc={onSelectDoc}
+              docDropProps={docDropProps}
+              mergeHoverId={mergeHoverId}
             />
           ))}
 
@@ -243,10 +252,13 @@ const FolderTreeItem: React.FC<FolderTreeItemProps> = ({
               key={d.id}
               draggable
               onDragStart={(e) => handleDocDragStart(e, d.id)}
+{...docDropProps(d.id)}
               onClick={() => onSelectDoc(d.id)}
               style={{ paddingLeft: `${(depth + 1) * 12 + 16}px` }}
               className={`w-full flex items-center gap-2 pr-2 py-1.5 rounded-btn text-left text-xs cursor-pointer border transition-colors ${
-                selectedDocId === d.id && subView === 'editor'
+                mergeHoverId === d.id
+                  ? 'border-accent bg-accent/15 ring-2 ring-accent/30 text-main'
+                  : selectedDocId === d.id && subView === 'editor'
                   ? 'bg-panel-elevated text-main font-semibold border-border/80'
                   : 'border-transparent text-main hover:bg-panel-elevated'
               }`}
@@ -378,6 +390,20 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
 
   // Drag and Drop state
   const [draggingDocId, setDraggingDocId] = useState<string | null>(null);
+
+  // Merging notes: dialog (picker/confirmation) and a toast with undo afterwards
+  const [mergeDialog, setMergeDialog] = useState<{ sourceId: string; targetId?: string } | null>(null);
+  const [mergeHoverId, setMergeHoverId] = useState<string | null>(null);
+  const [toast, setToast] = useState<{ text: string; actions: { label: string; run: () => void }[] } | null>(null);
+  const toastTimerRef = useRef<number | null>(null);
+  const showToast = useCallback((text: string, actions: { label: string; run: () => void }[] = []) => {
+    setToast({ text, actions });
+    if (toastTimerRef.current !== null) window.clearTimeout(toastTimerRef.current);
+    toastTimerRef.current = window.setTimeout(() => setToast(null), actions.length ? 10000 : 4000);
+  }, []);
+  useEffect(() => () => {
+    if (toastTimerRef.current !== null) window.clearTimeout(toastTimerRef.current);
+  }, []);
   const [draggingFolderPath, setDraggingFolderPath] = useState<string | null>(null);
   const [dragOverFolder, setDragOverFolder] = useState<string | null>(null);
 
@@ -674,6 +700,66 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
     setDraggingDocId(docId);
   };
 
+  const requestMerge = (sourceId: string, targetId?: string) => {
+    const source = docs.find((d) => d.id === sourceId);
+    if (!source?.isOwner) {
+      showToast('Nur eigene Notizen lassen sich in eine andere einfügen');
+      return;
+    }
+    setMergeDialog({ sourceId, targetId });
+  };
+
+  // Dropping a dragged note onto another note (instead of a folder) offers to merge them
+  const docDropProps = (targetId: string): React.HTMLAttributes<HTMLDivElement> => ({
+    onDragOver: (e) => {
+      if (!draggingDocId || draggingDocId === targetId) return;
+      e.preventDefault();
+      e.stopPropagation();
+      e.dataTransfer.dropEffect = 'move';
+      if (mergeHoverId !== targetId) setMergeHoverId(targetId);
+    },
+    onDragLeave: () => setMergeHoverId((cur) => (cur === targetId ? null : cur)),
+    onDrop: (e) => {
+      const sourceId = e.dataTransfer.getData('application/mindsift-doc-id') || draggingDocId;
+      if (!sourceId || sourceId === targetId) return;
+      e.preventDefault();
+      e.stopPropagation();
+      setDraggingDocId(null);
+      setMergeHoverId(null);
+      setDragOverFolder(null);
+      requestMerge(sourceId, targetId);
+    },
+  });
+
+  const handleMerged = async (result: MergeResult) => {
+    setMergeDialog(null);
+    triggerHaptic(20);
+    setDocs((prev) => prev.filter((d) => d.id !== result.sourceId));
+    setSelectedDocId(result.targetId);
+    setTargetBlockId(null);
+    setSubView('editor');
+    setMobileShowDetail(true);
+    setEditorNonce((n) => n + 1);
+    reloadTrash();
+    showToast(`„${result.sourceTitle || 'Notiz'}“ wurde angehängt`, [
+      {
+        label: 'Rückgängig',
+        run: async () => {
+          try {
+            await api.undoMerge(result);
+            await loadWorkspaceData();
+            setSelectedDocId(result.sourceId);
+            setEditorNonce((n) => n + 1);
+            reloadTrash();
+            showToast('Zusammenführen rückgängig gemacht');
+          } catch (err: any) {
+            showToast(err?.message || 'Rückgängig fehlgeschlagen');
+          }
+        },
+      },
+    ]);
+  };
+
   const handleFolderDragStart = (e: React.DragEvent, path: string) => {
     e.stopPropagation();
     e.dataTransfer.setData('application/mindsift-folder-path', path);
@@ -1348,13 +1434,16 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
                       key={`pin-${d.id}`}
                       draggable
                       onDragStart={(e) => handleDocDragStart(e, d.id)}
+{...docDropProps(d.id)}
                       onClick={() => {
                         setSelectedDocId(d.id);
                         setSubView('editor');
                         setMobileShowDetail(true);
                       }}
                       className={`w-full flex items-center gap-2 px-2 py-1.5 rounded-btn text-left text-xs cursor-pointer border transition-colors ${
-                        selectedDocId === d.id && subView === 'editor'
+                        mergeHoverId === d.id
+                          ? 'border-accent bg-accent/15 ring-2 ring-accent/30 text-main'
+                          : selectedDocId === d.id && subView === 'editor'
                           ? 'bg-panel-elevated text-main font-semibold border-border/80'
                           : 'border-transparent text-main hover:bg-panel-elevated'
                       }`}
@@ -1398,6 +1487,8 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
                         setSubView('editor');
                         setMobileShowDetail(true);
                       }}
+                      docDropProps={docDropProps}
+                      mergeHoverId={mergeHoverId}
                     />
                   ))}
 
@@ -1406,13 +1497,16 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
                       key={d.id}
                       draggable
                       onDragStart={(e) => handleDocDragStart(e, d.id)}
+{...docDropProps(d.id)}
                       onClick={() => {
                         setSelectedDocId(d.id);
                         setSubView('editor');
                         setMobileShowDetail(true);
                       }}
                       className={`w-full flex items-center gap-2 px-2 py-1.5 rounded-btn text-left text-xs cursor-pointer border transition-colors ${
-                        selectedDocId === d.id && subView === 'editor'
+                        mergeHoverId === d.id
+                          ? 'border-accent bg-accent/15 ring-2 ring-accent/30 text-main'
+                          : selectedDocId === d.id && subView === 'editor'
                           ? 'bg-panel-elevated text-main font-semibold border-border/80'
                           : 'border-transparent text-main hover:bg-panel-elevated'
                       }`}
@@ -1527,6 +1621,7 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
             onSelectTag={handleSelectTag}
             onWikiLinkClick={handleWikiLinkClick}
             onOpenDocBlock={handleJumpToDocBlock}
+            onRequestMerge={(d) => setMergeDialog({ sourceId: d.id })}
             onBackMobile={() => setMobileShowDetail(false)}
             onOpenTask={onOpenTask}
             onSaveStatusChange={(status, err) => {
@@ -2251,6 +2346,38 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
         </FormSheet>
       )}
 
+      {mergeDialog && (
+        <MergeDialog
+          docs={docs}
+          sourceId={mergeDialog.sourceId}
+          targetId={mergeDialog.targetId}
+          onClose={() => setMergeDialog(null)}
+          onMerged={(r) => void handleMerged(r)}
+        />
+      )}
+
+      {toast && (
+        <div
+          role="status"
+          className="fixed left-1/2 -translate-x-1/2 bottom-[calc(5.5rem+env(safe-area-inset-bottom))] sm:bottom-6 z-50 max-w-[calc(100vw-1.5rem)] px-3 py-2 rounded-card text-xs text-main bg-panel-elevated border border-border shadow-panel flex items-center gap-3"
+        >
+          <span>{toast.text}</span>
+          {toast.actions.map((a) => (
+            <button
+              key={a.label}
+              type="button"
+              onClick={() => {
+                setToast(null);
+                a.run();
+              }}
+              className="font-semibold text-accent hover:underline min-h-[44px] sm:min-h-0 shrink-0"
+            >
+              {a.label}
+            </button>
+          ))}
+        </div>
+      )}
+
       {/* Unified Sharing Modal (Strictly Read-Only for Docs and Folders) */}
       {(shareModalDoc || shareModalFolder) && (
         <UnifiedShareModal