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/static/js/admin-page.js

@@ -62,7 +62,7 @@ document.addEventListener('DOMContentLoaded', function() {
 
     function openUserModal(user = null) {
         document.getElementById('user-modal-title').textContent = user ? 'Edit User' : 'Create User';
-        document.getElementById('user-id').value = user ? user.id : '';
+        document.getElementById('user-id').value = (user && user.id) ? user.id : '';
         document.getElementById('user-username').value = user ? user.username : '';
         document.getElementById('user-username').disabled = !!user;
         document.getElementById('user-email').value = user ? user.email : '';
@@ -115,6 +115,7 @@ document.addEventListener('DOMContentLoaded', function() {
         saveBtn.textContent = 'Saving...';
 
         const id = document.getElementById('user-id').value;
+        const isEdit = id && id !== '' && id !== 'undefined';
         const data = {
             username: document.getElementById('user-username').value,
             email: document.getElementById('user-email').value,
@@ -126,7 +127,7 @@ document.addEventListener('DOMContentLoaded', function() {
         if (password) data.password = password;
 
         try {
-            if (id) {
+            if (isEdit) {
                 await LinkSync.updateUser(id, data);
             } else {
                 if (!password) {