Implement Login page with PocketBase auth (P1.6)

Add functional login page with email/password form:
- Client component with controlled form inputs
- PocketBase authentication integration
- Error handling with visual feedback
- Loading states (disabled inputs, button text change)
- Form validation (prevents empty submissions)
- Redirect to dashboard on successful login

Test infrastructure improvements:
- Add @testing-library/jest-dom for DOM matchers
- Add global test setup with cleanup between tests
- Configure vitest.config.ts with setupFiles

14 new tests covering form rendering, auth flow, error
handling, and validation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -30,6 +30,7 @@
     "@biomejs/biome": "2.3.11",
     "@tailwindcss/postcss": "^4",
     "@testing-library/dom": "^10.4.1",
+    "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.1",
     "@types/node": "^20",
     "@types/node-cron": "^3.0.11",