feat(workspace): backlinks at the end of a note

GET /documents/{id}/backlinks lists the notes the user can open whose
blocks link [[Title]] (also |alias and #section, case-insensitive),
excluding the note itself and the trash. The collapsible "Verlinkt auf
diese Notiz" section shows each note with folder and the linking bullets;
a click jumps straight to the bullet.

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

frontend/src/features/workspace/BacklinksSection.tsx

@@ -0,0 +1,118 @@
+import React, { useEffect, useState } from 'react';
+import { ChevronDown, ChevronRight, Folder, Link2 } from 'lucide-react';
+import { Backlink } from '../../lib/types';
+import { api } from '../../lib/api';
+
+interface BacklinksSectionProps {
+  docId: string;
+  title: string;
+  onOpen: (docId: string, blockId?: string) => void;
+}
+
+const COLLAPSED_KEY = 'mindsift:backlinks_collapsed';
+
+// Shows the [[Wiki-Link]] parts of a block in the accent color, the rest as plain text
+const Snippet: React.FC<{ text: string }> = ({ text }) => (
+  <>
+    {text.split(/(\[\[[^\]]*\]\])/g).map((part, i) =>
+      part.startsWith('[[') && part.endsWith(']]') ? (
+        <span key={i} className="text-accent font-medium">
+          {part}
+        </span>
+      ) : (
+        <React.Fragment key={i}>{part}</React.Fragment>
+      )
+    )}
+  </>
+);
+
+// "Verlinkt auf diese Notiz" at the end of a note. Hidden while nothing links here; reloads shortly
+// after the title changes, since links are matched by title.
+export const BacklinksSection: React.FC<BacklinksSectionProps> = ({ docId, title, onOpen }) => {
+  const [links, setLinks] = useState<Backlink[]>([]);
+  const [collapsed, setCollapsed] = useState<boolean>(() => {
+    try {
+      return localStorage.getItem(COLLAPSED_KEY) === 'true';
+    } catch {
+      return false;
+    }
+  });
+
+  useEffect(() => {
+    let cancelled = false;
+    const timer = setTimeout(() => {
+      api
+        .getBacklinks(docId)
+        .then((res) => !cancelled && setLinks(res))
+        .catch(() => !cancelled && setLinks([]));
+    }, 400);
+    return () => {
+      cancelled = true;
+      clearTimeout(timer);
+    };
+  }, [docId, title]);
+
+  if (links.length === 0) return null;
+
+  const toggle = () => {
+    setCollapsed((prev) => {
+      try {
+        localStorage.setItem(COLLAPSED_KEY, String(!prev));
+      } catch {}
+      return !prev;
+    });
+  };
+
+  return (
+    <section className="mt-10 pt-4 border-t border-border" aria-label="Rückverweise">
+      <button
+        type="button"
+        onClick={toggle}
+        aria-expanded={!collapsed}
+        className="w-full flex items-center gap-1.5 min-h-[44px] sm:min-h-0 py-1 text-xs font-semibold text-muted hover:text-main cursor-pointer"
+      >
+        {collapsed ? <ChevronRight className="w-3.5 h-3.5" /> : <ChevronDown className="w-3.5 h-3.5" />}
+        <Link2 className="w-3.5 h-3.5 text-accent" />
+        <span>Verlinkt auf diese Notiz ({links.length})</span>
+      </button>
+      {!collapsed && (
+        <ul className="mt-2 space-y-2">
+          {links.map((l) => (
+            <li key={l.docId} className="p-3 bg-panel border border-border rounded-card">
+              <button
+                type="button"
+                onClick={() => onOpen(l.docId)}
+                className="flex items-center gap-2 min-w-0 max-w-full text-left cursor-pointer group"
+              >
+                <span className="shrink-0">{l.icon || '📝'}</span>
+                <span className="text-sm font-semibold text-main truncate group-hover:text-accent">
+                  {l.title || 'Unbenannte Notiz'}
+                </span>
+                {l.folderPath && (
+                  <span className="hidden sm:inline-flex items-center gap-1 text-[11px] text-faint min-w-0">
+                    <Folder className="w-3 h-3 shrink-0" />
+                    <span className="truncate">{l.folderPath}</span>
+                  </span>
+                )}
+              </button>
+              <ul className="mt-1.5 space-y-1">
+                {l.blocks.map((b) => (
+                  <li key={b.id}>
+                    <button
+                      type="button"
+                      onClick={() => onOpen(l.docId, b.id)}
+                      className="w-full text-left text-xs text-muted hover:text-main px-2 py-1.5 rounded-btn hover:bg-panel-elevated border-l-2 border-border line-clamp-3 break-words cursor-pointer"
+                    >
+                      <Snippet text={b.content} />
+                    </button>
+                  </li>
+                ))}
+              </ul>
+              {!!l.more && <p className="mt-1 text-[11px] text-faint">… und {l.more} weitere Stellen</p>}
+            </li>
+          ))}
+        </ul>
+      )}
+    </section>
+  );
+};

frontend/src/features/workspace/DocumentEditor.tsx

@@ -41,12 +41,14 @@ export interface DocumentEditorProps {
   onOpenShareModal: (doc: Document) => void;
   onSelectTag: (tag: string) => void;
   onWikiLinkClick?: (pageTitle: string) => void;
+  onOpenDocBlock?: (docId: string, blockId?: string) => void;
   onBackMobile?: () => void;
   onOpenTask?: (taskId: string) => void;
   onSaveStatusChange?: (status: DocSaveStatus, errorMsg?: string) => void;
 }
 
 import { EmojiPickerModal } from './EmojiPickerModal';
+import { BacklinksSection } from './BacklinksSection';
 
 function ensureNonEmptyBlocks(docId: string, blocks?: DocumentBlock[]): DocumentBlock[] {
   if (!blocks || blocks.length === 0) {
@@ -182,6 +184,7 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
   onDocMetadataChange,
   onDeleteDoc,
   onRestoreDoc,
+  onOpenDocBlock,
   onOpenShareModal,
   onSelectTag,
   onWikiLinkClick,
@@ -1070,6 +1073,8 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
               onUndo={handleUndo}
               onRedo={handleRedo}
             />
+
+            {onOpenDocBlock && !inTrash && <BacklinksSection docId={docId} title={title} onOpen={onOpenDocBlock} />}
           </>
         )}
       </div>

frontend/src/features/workspace/WorkspaceView.tsx

@@ -1583,6 +1583,7 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
             onOpenShareModal={openDocShareModal}
             onSelectTag={handleSelectTag}
             onWikiLinkClick={handleWikiLinkClick}
+            onOpenDocBlock={handleJumpToDocBlock}
             onBackMobile={() => setMobileShowDetail(false)}
             onOpenTask={onOpenTask}
             onSaveStatusChange={(status, err) => {