feat(05-02): add search and note widgets with tests

- SearchWidget: provider dropdown, text input, button; opens search in new tab via window.open (D-14/D-15)
- NoteWidget: MDEditor with compact toolbar, debounced autosave (1500ms), AbortController for in-flight cancellation (D-16/D-17/D-18)
- rehype-sanitize enabled for Markdown XSS prevention (T-05-05)
- Widget registry updated with wireSearchWidget/wireNoteWidget (no more placeholders)
- dashboard-api.ts: added fetchSearchProviders, addSearchProvider, removeSearchProvider, signal support on updateWidgetConfig
- 9 new tests passing (search: 5, note: 4)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/package.json

@@ -10,6 +10,7 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
+    "@uiw/react-md-editor": "4.1.1",
     "jose": "^6.2.3",
     "next": "^15.3.0",
     "next-intl": "^4.13.0",
@@ -18,6 +19,7 @@
     "react-dom": "^19.0.0",
     "react-grid-layout": "2.2.3",
     "react-resizable": "^4.0.2",
+    "rehype-sanitize": "^6.0.0",
     "zod": "^4.4.3",
     "zustand": "^5.0.14"
   },