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/routes/index.tsx

@@ -1,10 +1,13 @@
 import { createFileRoute } from '@tanstack/react-router';
+import { trpc } from '@/lib/trpc';
 
 export const Route = createFileRoute('/')({
   component: HomePage,
 });
 
 function HomePage() {
+  const pingQuery = trpc.health.ping.useQuery();
+
   return (
     <div className="flex flex-col items-center justify-center h-full gap-4">
       <div className="flex items-center gap-3">
@@ -27,6 +30,11 @@ function HomePage() {
       <p className="text-muted-foreground text-sm">
         Adiuva is ready. Start building.
       </p>
+      {pingQuery.data && (
+        <p className="text-xs text-muted-foreground">
+          tRPC IPC bridge: {pingQuery.data}
+        </p>
+      )}
     </div>
   );
 }