Adopt React Query and live updates across all UI pages

Replace custom useApi/useAgents hooks with @tanstack/react-query.
Add LiveUpdatesProvider for WebSocket-driven cache invalidation.
Add queryKeys module for centralized cache key management. Rework
all pages and dialogs to use React Query mutations and queries.
Improve CompanyContext with query-based data fetching.

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

ui/src/hooks/useAgents.ts

@@ -1,11 +0,0 @@
-import { useCallback } from "react";
-import { agentsApi } from "../api/agents";
-import { useApi } from "./useApi";
-
-export function useAgents(companyId: string | null) {
-  const fetcher = useCallback(() => {
-    if (!companyId) return Promise.resolve([]);
-    return agentsApi.list(companyId);
-  }, [companyId]);
-  return useApi(fetcher);
-}

ui/src/hooks/useApi.ts

@@ -1,21 +0,0 @@
-import { useState, useEffect, useCallback } from "react";
-
-export function useApi<T>(fetcher: () => Promise<T>) {
-  const [data, setData] = useState<T | null>(null);
-  const [error, setError] = useState<Error | null>(null);
-  const [loading, setLoading] = useState(true);
-
-  const load = useCallback(() => {
-    setLoading(true);
-    fetcher()
-      .then(setData)
-      .catch(setError)
-      .finally(() => setLoading(false));
-  }, [fetcher]);
-
-  useEffect(() => {
-    load();
-  }, [load]);
-
-  return { data, error, loading, reload: load };
-}