Store multiple documents in IndexedDB with a document list

Documents now live in an IndexedDB library instead of a single
localStorage slot: each has its own ?doc=<id> URL, a title (editable in
the toolbar, used for export file names) and appears in Datei ›
Dokumente (open, rename, duplicate, delete). New/sample/import create
new documents instead of replacing the open one. Tabs stay in step via
BroadcastChannel. The old localStorage document is migrated once.

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

package.json

@@ -26,6 +26,7 @@
     "@vue/eslint-config-typescript": "^14.1.3",
     "eslint": "^9.15.0",
     "eslint-plugin-vue": "^9.31.0",
+    "fake-indexeddb": "^6.2.5",
     "jsdom": "^25.0.1",
     "prettier": "^3.3.3",
     "typescript": "~5.6.3",