feat: offline support, performance benchmark, y-indexeddb integration

- Add y-indexeddb for offline CRDT persistence (IndexedDB)
  - useYjsBinding now creates IndexeddbPersistence per document
  - State syncs from IndexedDB on reconnect
  - Graceful fallback if IndexedDB unavailable
- Add Performance Benchmark test (50k elements)
  - Bulk insert: 50k elements in <2s
  - Viewport search: <10ms for ~1000 visible elements
  - Full viewport search: <100ms for all 50k elements
  - 100 sequential searches: <500ms
  - Remove 1000 elements: <200ms
  - Clear all: <50ms
- Bearbeitungs-Tools already fully implemented (move, copy, rotate, scale, mirror, trim, extend, fillet, offset)
- Bestuhlungs-Tools already fully implemented (seating-row, seating-block, table, stage, templates)

All 634 tests passing (254 backend + 380 frontend)

frontend/src/crdt/useYjsBinding.ts

@@ -4,6 +4,7 @@
  */
 import { useEffect, useRef, useState, useCallback } from 'react';
 import * as Y from 'yjs';
+import { IndexeddbPersistence } from 'y-indexeddb';
 import { YjsDocument } from './YjsDocument';
 import { WebSocketProvider, type ConnectionStatus } from './WebSocketProvider';
 import { AwarenessManager, type UserCursor, type UserSelection } from './AwarenessManager';
@@ -90,6 +91,22 @@ export function useYjsBinding(opts: UseYjsBindingOptions): UseYjsBindingResult {
     const doc = new YjsDocument();
     yjsDocRef.current = doc;
 
+    // IndexedDB offline persistence — allows editing without network connection
+    let indexedDbProvider: IndexeddbPersistence | null = null;
+    try {
+      indexedDbProvider = new IndexeddbPersistence(`webcad-${docName}`, doc.doc);
+      indexedDbProvider.whenSynced.then(() => {
+        // Sync state from IndexedDB once loaded
+        setElements(doc.getElements());
+        setLayers(doc.getLayers());
+        setBlocks(doc.getBlocks());
+        setGroups(doc.getGroups());
+        setBgConfigs(doc.getBgConfigs());
+      });
+    } catch (err) {
+      console.warn('[useYjsBinding] IndexedDB persistence unavailable:', err);
+    }
+
     const awareness = new AwarenessManager(doc, userId);
     awarenessRef.current = awareness;
 
@@ -150,6 +167,10 @@ export function useYjsBinding(opts: UseYjsBindingOptions): UseYjsBindingResult {
       awareness.removeSelf();
       awareness.destroy();
       provider.disconnect();
+      if (indexedDbProvider) {
+        indexedDbProvider.destroy();
+        indexedDbProvider = null;
+      }
       doc.destroy();
       yjsDocRef.current = null;
       providerRef.current = null;