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/vitest.config.ts

@@ -16,7 +16,7 @@ export default defineConfig({
     css: false,
     server: {
       deps: {
-        inline: ['react-grid-layout', 'react-resizable'],
+        inline: ['react-grid-layout', 'react-resizable', '@uiw/react-md-editor'],
       },
     },
   },