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/main/index.ts

@@ -1,14 +1,16 @@
 import { app, BrowserWindow } from 'electron';
 import path from 'node:path';
 import started from 'electron-squirrel-startup';
+import { createIPCHandler } from 'electron-trpc/main';
 import { initDb } from './db';
+import { appRouter } from './router';
 
 // Handle creating/removing shortcuts on Windows when installing/uninstalling.
 if (started) {
   app.quit();
 }
 
-const createWindow = () => {
+const createWindow = (): BrowserWindow => {
   // Create the browser window.
   const mainWindow = new BrowserWindow({
     width: 1280,
@@ -36,6 +38,8 @@ const createWindow = () => {
   if (process.env.NODE_ENV === 'development') {
     mainWindow.webContents.openDevTools();
   }
+
+  return mainWindow;
 };
 
 // This method will be called when Electron has finished
@@ -43,7 +47,8 @@ const createWindow = () => {
 // Some APIs can only be used after this event occurs.
 app.on('ready', () => {
   initDb();
-  createWindow();
+  const win = createWindow();
+  createIPCHandler({ router: appRouter, windows: [win] });
 });
 
 // Quit when all windows are closed, except on macOS. There, it's common