Add Cmd+K command palette for searching executions and agents

Backend: add routeId, agentId, processorType filter fields to SearchRequest
and ClickHouseSearchEngine. Expand global text search to match route_id and
agent_id columns.

Frontend: new command palette component (portal overlay, Zustand store,
TanStack Query search hook with 300ms debounce, filter chip parsing,
keyboard navigation, scope tabs). Search bar in SearchFilters and TopNav
now open the palette. Selecting a result writes filters to the execution
search store to drive the results table.

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

ui/src/api/schema.d.ts

@@ -122,6 +122,9 @@ export interface SearchRequest {
   textInBody?: string | null;
   textInHeaders?: string | null;
   textInErrors?: string | null;
+  routeId?: string | null;
+  agentId?: string | null;
+  processorType?: string | null;
   offset?: number;
   limit?: number;
 }