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>

vitest.config.ts

@@ -14,5 +14,6 @@ export default defineConfig({
   test: {
     environment: "jsdom",
     include: ["src/**/*.test.{ts,tsx}"],
+    setupFiles: ["./src/test-setup.ts"],
   },
 });