perf: move PouchDB/IndexedDB operations to a Web Worker

All database reads and writes now run off the main thread via a
dedicated worker, eliminating IndexedDB overhead from the frame budget.

- Create data.worker.ts exposing the Data class via Comlink
- Refactor data.ts to export a Comlink-wrapped proxy and a standalone
  generateId() pure function (workers can't expose sync methods cleanly)
- Update all 10 call sites to import generateId directly instead of
  calling data.generateId()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -33,6 +33,7 @@
     "@vueuse/core": "^13.6.0",
     "@vueuse/router": "^13.6.0",
     "arktype": "^2.1.29",
+    "comlink": "^4.4.2",
     "date-fns": "^4.1.0",
     "events": "^3.3.0",
     "font-color-contrast": "^11.1.0",