feat: migrate test runner from Bun to Vitest

- Replace bun:test with Vitest across all 22 test files
- Create @sofa/test package with shared setup and DB test helpers
  - setup.ts: vi.mock for @sofa/db/client, Bun.randomUUIDv7 polyfill
  - db.ts: in-memory SQLite via better-sqlite3, seed helpers
- Add per-project vitest configs (packages/core, apps/web)
- Add root vitest.config.ts with projects and v8 coverage
- Set up @vitest/browser + Playwright for web component tests
- Move validateBackupDatabase from core/backup.ts to @sofa/db/client
- Derive required backup tables from schema instead of hard-coded list
- Update CI workflow with Playwright install and Codecov upload
- Update CLAUDE.md documentation for Vitest

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -23,6 +23,9 @@ jobs:
       - name: Install dependencies
         run: bun install --frozen-lockfile
 
+      - name: Install Playwright browsers
+        run: bunx playwright install chromium
+
       - name: Generate Expo types
         run: bunx expo customize tsconfig.json
         working-directory: ./apps/native
@@ -37,4 +40,9 @@ jobs:
         run: bunx turbo run check-types
 
       - name: Run tests
-        run: bunx turbo run test -- --coverage --coverage-reporter=lcov
+        run: bunx turbo run test -- --coverage
+
+      - name: Upload coverage reports to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}