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/src/components/dashboard/widget-registry.tsx

@@ -170,7 +170,7 @@ export const WIDGET_REGISTRY: Record<WidgetType, WidgetDefinition> = {
     descriptionKey: 'search.description',
     icon: SearchIcon,
     ...WIDGET_CONSTRAINTS.search,
-    component: PlaceholderWidget,
+    component: PlaceholderWidget, // Replaced via wireSearchWidget()
   },
   calendar: {
     type: 'calendar',
@@ -186,12 +186,12 @@ export const WIDGET_REGISTRY: Record<WidgetType, WidgetDefinition> = {
     descriptionKey: 'note.description',
     icon: NoteIcon,
     ...WIDGET_CONSTRAINTS.note,
-    component: PlaceholderWidget,
+    component: PlaceholderWidget, // Replaced via wireNoteWidget()
   },
 };
 
-// Wire actual ClockWidget lazily to avoid circular deps
-// (import is deferred so widget-registry can be imported by tests without
+// Wire actual widget components lazily to avoid circular deps
+// (imports are deferred so widget-registry can be imported by tests without
 // pulling in the entire React tree)
 let clockWired = false;
 export function wireClockWidget(
@@ -202,3 +202,23 @@ export function wireClockWidget(
     clockWired = true;
   }
 }
+
+let searchWired = false;
+export function wireSearchWidget(
+  component: ComponentType<WidgetProps>,
+) {
+  if (!searchWired) {
+    WIDGET_REGISTRY.search.component = component;
+    searchWired = true;
+  }
+}
+
+let noteWired = false;
+export function wireNoteWidget(
+  component: ComponentType<WidgetProps>,
+) {
+  if (!noteWired) {
+    WIDGET_REGISTRY.note.component = component;
+    noteWired = true;
+  }
+}