feat(notes): show block locks and presence, conflict copies with accept/discard, undo/redo

Phase 3 of the collaboration plan:
- Blocks held by someone else show a lock with the name and are read-only;
  Enter in such a block adds a new block below. Delete, merge, indent and
  drag are not started when a involved block (or one in its branch) is
  locked; a short notice explains why (fixes FLAW-25).
- Presence: the editor header lists who else has the note open.
- Conflict copies are highlighted ("Konflikt – Version von …") with
  Übernehmen (replaces the original or the title) and Verwerfen. Both check
  the locks of the original and of the copy first.
- Undo/redo of own changes (Ctrl/Cmd+Z, Ctrl+Shift+Z, Ctrl+Y, beforeinput
  history events): typing is grouped per block until a 1 s pause, up to 200
  steps per tab; a step is skipped with a notice if someone else changed or
  holds the block meanwhile. New clearConflict props patch on the server.
- New blocks get focus synchronously on mount (fixes FLAW-26).
- Fix: after reopening a note from local storage the engine polled every
  30 s instead of 3 s until the next change, because the delay was decided
  before the first response told it the note is shared.
- Tests: undo/conflict engine tests, lock/undo/conflict UI tests and
  e2e/lock.e2e.mjs (two browsers); all three browser tests pass against a
  freshly built image.

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

frontend/src/features/workspace/DocumentEditor.tsx

@@ -405,6 +405,50 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
     }
   }, [engineState?.status, engineState?.errorMessage]);
 
+  // Blocks other users are editing right now (blockId -> name)
+  const lockedBlocks = useMemo(() => {
+    const m = new Map<string, string>();
+    for (const l of engineState?.lockedByOthers || []) m.set(l.blockId, l.name || 'Jemand');
+    return m;
+  }, [engineState?.lockedByOthers]);
+
+  const otherPeople = useMemo(
+    () => (engineState?.presence || []).filter((p) => p.userId !== currentUser?.id),
+    [engineState?.presence, currentUser?.id]
+  );
+
+  // Short hint at the bottom of the editor (blocked actions, skipped undo)
+  const [notice, setNotice] = useState<string | null>(null);
+  const noticeTimerRef = useRef<number | null>(null);
+  const showNotice = useCallback((message: string) => {
+    setNotice(message);
+    if (noticeTimerRef.current !== null) window.clearTimeout(noticeTimerRef.current);
+    noticeTimerRef.current = window.setTimeout(() => setNotice(null), 3500);
+  }, []);
+  useEffect(() => () => {
+    if (noticeTimerRef.current !== null) window.clearTimeout(noticeTimerRef.current);
+  }, []);
+
+  const handleLockedAction = useCallback(
+    (holder: string) => showNotice(`${holder} bearbeitet diesen Punkt gerade – bitte kurz warten`),
+    [showNotice]
+  );
+  const handleUndo = useCallback(() => {
+    const res = engineRef.current?.undo();
+    if (res?.message) showNotice(res.message);
+  }, [showNotice]);
+  const handleRedo = useCallback(() => {
+    const res = engineRef.current?.redo();
+    if (res?.message) showNotice(res.message);
+  }, [showNotice]);
+  const handleResolveConflict = useCallback(
+    (blockId: string, accept: boolean) => {
+      const res = engineRef.current?.resolveConflict(blockId, accept);
+      if (res?.message) showNotice(res.message);
+    },
+    [showNotice]
+  );
+
   // Every local block change goes to the engine, which turns it into operations
   const applyLocalBlocks = useCallback(
     (update: DocumentBlock[] | ((prev: DocumentBlock[]) => DocumentBlock[])) => {
@@ -722,6 +766,16 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
 
         {/* Right Action Buttons */}
         <div className="flex items-center gap-1.5">
+          {/* Who else has this note open */}
+          {otherPeople.length > 0 && (
+            <span
+              className="hidden sm:inline-flex items-center gap-1 px-2 py-0.5 rounded-btn text-[11px] text-muted bg-panel border border-border"
+              title="Hat diese Notiz gerade geöffnet"
+            >
+              <Users className="w-3 h-3 text-accent" />
+              {otherPeople.map((p) => p.name).join(', ')}
+            </span>
+          )}
           {/* Sharing Section */}
           {doc && (() => {
             const isOwner = doc.isOwner !== false && (doc.userId === currentUser?.id || !doc.userId || currentUser?.role === 'admin');
@@ -907,6 +961,11 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
                 onWikiLinkClick={onWikiLinkClick}
                 initialFocusedBlockId={initialFocusedBlockId}
                 onFocusedBlockChange={(id) => engineRef.current?.focusBlock(id)}
+                lockedBlocks={lockedBlocks}
+                onLockedAction={handleLockedAction}
+                onResolveConflict={handleResolveConflict}
+                onUndo={handleUndo}
+                onRedo={handleRedo}
               />
             )}
           </div>
@@ -1014,6 +1073,12 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
                   e.target.style.height = `${e.target.scrollHeight}px`;
                 }}
                 onKeyDown={(e) => {
+                  if ((e.ctrlKey || e.metaKey) && !e.altKey && ['z', 'y'].includes(e.key.toLowerCase())) {
+                    e.preventDefault();
+                    if (e.key.toLowerCase() === 'y' || e.shiftKey) handleRedo();
+                    else handleUndo();
+                    return;
+                  }
                   if (e.key === 'Enter' && !e.shiftKey) {
                     e.preventDefault();
                     outlinerTreeRef.current?.focusFirstBlock();
@@ -1044,10 +1109,24 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
               onWikiLinkClick={onWikiLinkClick}
               initialFocusedBlockId={initialFocusedBlockId}
               onFocusedBlockChange={(id) => engineRef.current?.focusBlock(id)}
+              lockedBlocks={lockedBlocks}
+              onLockedAction={handleLockedAction}
+              onResolveConflict={handleResolveConflict}
+              onUndo={handleUndo}
+              onRedo={handleRedo}
             />
           </>
         )}
       </div>
+
+      {notice && (
+        <div
+          role="status"
+          className="fixed bottom-6 left-1/2 -translate-x-1/2 z-50 max-w-[90vw] px-3 py-2 rounded-card text-xs text-main bg-panel-elevated border border-border shadow-panel"
+        >
+          {notice}
+        </div>
+      )}
     </div>
   );
 };