feat(workspace): sidebar search opens the global search

The sidebar filter only matched titles and tags, never the bullets. It
is replaced by a search-field-like button that opens the command palette
(full-text search over all notes and bullets).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1086,6 +1086,7 @@ export const App: React.FC = () => {
                 initialBlockId={selectedBlockId}
                 onStatsChange={refreshAll}
                 onOpenTask={handleOpenTask}
+                onOpenSearch={() => setShowCommandPalette(true)}
               />
             )}