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/lib/queryKeys.ts

@@ -0,0 +1,33 @@
+export const queryKeys = {
+  companies: {
+    all: ["companies"] as const,
+    detail: (id: string) => ["companies", id] as const,
+  },
+  agents: {
+    list: (companyId: string) => ["agents", companyId] as const,
+    detail: (id: string) => ["agents", "detail", id] as const,
+  },
+  issues: {
+    list: (companyId: string) => ["issues", companyId] as const,
+    detail: (id: string) => ["issues", "detail", id] as const,
+    comments: (issueId: string) => ["issues", "comments", issueId] as const,
+  },
+  projects: {
+    list: (companyId: string) => ["projects", companyId] as const,
+    detail: (id: string) => ["projects", "detail", id] as const,
+  },
+  goals: {
+    list: (companyId: string) => ["goals", companyId] as const,
+    detail: (id: string) => ["goals", "detail", id] as const,
+  },
+  approvals: {
+    list: (companyId: string, status?: string) =>
+      ["approvals", companyId, status] as const,
+  },
+  dashboard: (companyId: string) => ["dashboard", companyId] as const,
+  activity: (companyId: string) => ["activity", companyId] as const,
+  costs: (companyId: string) => ["costs", companyId] as const,
+  heartbeats: (companyId: string, agentId?: string) =>
+    ["heartbeats", companyId, agentId] as const,
+  org: (companyId: string) => ["org", companyId] as const,
+};