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-lock.json

@@ -19,6 +19,7 @@
         "@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",
@@ -2885,6 +2886,16 @@
         "node": ">=12.0.0"
       }
     },
+    "node_modules/fake-indexeddb": {
+      "version": "6.2.5",
+      "resolved": "https://registry.npmjs.org/fake-indexeddb/-/fake-indexeddb-6.2.5.tgz",
+      "integrity": "sha512-CGnyrvbhPlWYMngksqrSSUT1BAVP49dZocrHuK0SvtR0D5TMs5wP0o3j7jexDJW01KSadjBp1M/71o/KR3nD1w==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/fast-deep-equal": {
       "version": "3.1.3",
       "resolved": "https://registry.npmjs.org/fast-deep-equal/-/fast-deep-equal-3.1.3.tgz",