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>

prd.json

@@ -49,8 +49,8 @@
         "Typecheck passes"
       ],
       "priority": 3,
-      "passes": false,
-      "notes": ""
+      "passes": true,
+      "notes": "Completed: electron-trpc IPC bridge, appRouter with stub routers for all 7 domains (health, clients, projects, tasks, checkpoints, notes, ai), renderer TRPCProvider+QueryClientProvider, health.ping returns 'pong' displayed in HomePage, Zod validates all procedure inputs"
     },
     {
       "id": "US-004",