feat: add web UI, query engine, session management, and 20 E2E tests

- Web UI: login, dashboard, links CRUD, collections, API keys, admin pages
- Query engine: AND/OR/XOR with field filters, tag search, preview endpoint
- Session management: token expiry detection, 401 interceptor, expiry banner
- Links search: tags included, multi-word AND, query mode with set operations
- Collections: static/dynamic, query builder with preview, public tree view
- Save as Collection: convert search results (static) or query (dynamic)
- Dashboard stats: resilient loading with allSettled pattern
- Login page: redesigned with public collections tree view
- Bug fix: query executor None fields crash (notes/description/url/title)
- E2E tests: 20 Playwright tests covering all critical user flows
- All 104 tests passing (84 unit/integration + 20 E2E)

LinkSyncServer/TODOs.txt

@@ -75,7 +75,24 @@
 - [x] Integration tests with TestClient
 - [x] Test configuration (tests/conftest.py)
 - [x] pytest.ini in pyproject.toml
-- [x] All 42 tests passing
+- [x] All 84 tests passing
+- [x] E2E tests with Playwright (20+ tests)
+- [x] Session management tests
+- [x] Search mode tests (simple, query, multi-word)
+- [x] Collection query builder tests
+- [x] Save as Collection tests
+
+## Web Interface
+- [x] Login page with session expiry handling
+- [x] Dashboard with stats and quick actions
+- [x] Links page with dual search modes
+- [x] Collections page with query builder
+- [x] API Keys page with CRUD
+- [x] Admin page with user management
+- [x] Save as Collection feature
+- [x] Token expiry detection and redirect
+- [x] Session expiry warnings
+- [x] Graceful error handling
 
 ## Documentation
 - [x] API reference (via /api/docs OpenAPI)
@@ -83,6 +100,8 @@
 - [x] Developer guide (AGENTS.md, design.md)
 - [x] Deployment guide (README.md)
 - [x] Query syntax reference (README.md)
+- [x] UI/UX design documentation (design.md)
+- [x] Implementation tasks (tasks.md)
 
 ## Security
 - [x] Password hashing (bcrypt with cost factor 12)