fix(canvas): sanitize snapshot session.currentPageId before loadSnapshot — stale snapshots with missing pages caused currentPageId crash

- Pre-load: if session.currentPageId not in snapshot pages, reset to first available page
- Post-load: if store instance.currentPageId still invalid, clear store for fresh start
- On loadSnapshot error: clear store instead of silently failing with corrupt state
- Root cause: teacher1 had a saved snapshot with currentPageId pointing to a deleted page

src/services/tldraw/snapshotService.ts

@@ -1,5 +1,5 @@
 // External imports
-import { TLStore, getSnapshot, Editor, loadSnapshot } from '@tldraw/tldraw';
+import { TLStore, getSnapshot, Editor, loadSnapshot, TLINSTANCE_ID } from '@tldraw/tldraw';
 import logger from '../../debugConfig';
 import { SharedStoreService } from './sharedStoreService';
 
@@ -109,17 +109,35 @@ export class NavigationSnapshotService {
                 return;
             }
 
+            // Sanitize session.currentPageId against pages actually in the document
+            const docStore = (snap.document as { store?: Record<string, { typeName: string; id: string }> })?.store ?? {};
+            const pageIds = Object.values(docStore).filter((r) => (r as { typeName: string }).typeName === 'page').map((r) => r.id);
+            const session = snap.session as { currentPageId?: string } | undefined;
+            if (session?.currentPageId && pageIds.length > 0 && !pageIds.includes(session.currentPageId)) {
+                logger.warn('snapshot-service', '⚠️ session.currentPageId not in snapshot pages — resetting to first page', {
+                    currentPageId: session.currentPageId, availablePages: pageIds
+                });
+                session.currentPageId = pageIds[0];
+            }
+
             const snapshotCopy = {
                 schemaVersion: snap.schemaVersion || (snap.document as { schema?: { schemaVersion?: unknown } })?.schema?.schemaVersion,
                 document: snap.document,
                 session: snap.session,
             };
 
+            const targetStore = editor ? editor.store : store;
             try {
-                if (editor) {
-                    loadSnapshot(editor.store, snapshotCopy as any);
-                } else {
-                    loadSnapshot(store, snapshotCopy as any);
+                loadSnapshot(targetStore, snapshotCopy as any);
+                // Post-load validation: ensure instance.currentPageId is still valid
+                const instance = targetStore.get(TLINSTANCE_ID);
+                if (instance) {
+                    const pages = targetStore.allRecords().filter((r) => (r as { typeName: string }).typeName === 'page');
+                    const pageValid = pages.some((p) => p.id === instance.currentPageId);
+                    if (!pageValid) {
+                        logger.warn('snapshot-service', '⚠️ Post-load: currentPageId invalid — clearing corrupt store', { currentPageId: instance.currentPageId });
+                        targetStore.clear();
+                    }
                 }
                 logger.debug('snapshot-service', '✅ Snapshot loaded successfully');
             } catch (err) {
@@ -128,7 +146,8 @@ export class NavigationSnapshotService {
                 if (isSchemaMigration) {
                     logger.debug('snapshot-service', 'ℹ️ Schema migration warning (non-critical)', { error: msg });
                 } else {
-                    logger.warn('snapshot-service', '⚠️ Unexpected loadSnapshot error', { error: msg });
+                    logger.warn('snapshot-service', '⚠️ Unexpected loadSnapshot error — clearing store to allow fresh start', { error: msg });
+                    targetStore.clear();
                 }
             }