fix: repair IDB schema lost after browser crash (closes #28)

[kasunben]

Summary

• Bumps openDB from version 1 to version 2 so onupgradeneeded re-runs for any v1 database that lost its object stores after a browser crash mid-upgrade
• Guards every createObjectStore call with !objectStoreNames.contains(...) so the upgrade is fully idempotent — existing intact databases are unaffected
• Removes window.location.reload() from the db.onversionchange handler; db.close() alone is sufficient to unblock future upgrades, and the forced reload was causing a reload cascade during Playwright navigations (root cause of the E2E reload-persistence test flake on this branch)

Root cause

On some browsers (notably Firefox), a crash during the very first onupgradeneeded execution leaves the IndexedDB at version 1 with no object stores. On every subsequent page load, openDB(siteId, 1) sees the DB already at v1 and skips onupgradeneeded. The first db.transaction('threads', 'readonly') call then throws:

DOMException: IDBDatabase.transaction: 'threads' is not a known object store name

The entire init promise chain falls into .catch, logging:

Annotate.js: IndexedDB unavailable — running in memory-only mode

In server-sync mode this is especially destructive — pullThreads, flushDirtyThreads, and setInterval are never wired up, so annotations appear to vanish on every page refresh.

Test plan

• npm run test:all — 82 unit + integration tests pass; 15 E2E tests pass (1 skipped: P2P relay, requires deployed relay)
• "annotations persist across page reload (IndexedDB)" — was consistently failing on this branch before the onversionchange reload removal; now passes
• Manual: open demo-sync-with-server.html, annotate, refresh — annotations persist
• Manual: simulate the crash scenario by opening DevTools → Application → IndexedDB, manually deleting the object stores from annotate-default (leaving the DB at v1 with no stores), then reload — annotations should work normally (stores recreated by v2 upgrade)

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