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>

codecov.yml

@@ -0,0 +1,11 @@
+# yaml-language-server: $schema=https://json.schemastore.org/codecov
+coverage:
+  status:
+    project:
+      default:
+        target: auto
+        informational: true
+    patch:
+      default:
+        target: auto
+        informational: true