feat: US-003 — electron-trpc IPC bridge and appRouter scaffold

- Install electron-trpc, @trpc/server, @trpc/client, @trpc/react-query, @tanstack/react-query, zod
- Create appRouter in src/main/router/index.ts with stub routers for: health, clients, projects, tasks, checkpoints, notes, ai
- health.ping procedure returns 'pong'
- All procedure inputs validated with Zod schemas
- Configure IPC handler in main process (createIPCHandler) and preload (exposeElectronTRPC)
- Create renderer trpc client in src/renderer/lib/trpc.ts with ipcLink
- Wrap app with TRPCProvider + QueryClientProvider in src/renderer/index.tsx
- Verify health.ping in HomePage component (shows 'tRPC IPC bridge: pong')
- Typecheck passes

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

src/renderer/lib/trpc.ts

@@ -0,0 +1,4 @@
+import { createTRPCReact } from '@trpc/react-query';
+import type { AppRouter } from '../../main/router';
+
+export const trpc = createTRPCReact<AppRouter>();