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+## Codebase Patterns
+- Vite configs use `.mts` extension (not `.ts`) to avoid ESM/CJS conflict with electron-forge's externalize-deps plugin
+- electron-trpc uses `exposeElectronTRPC()` in preload and `createIPCHandler({ router, windows })` in main; renderer uses `ipcLink()` from `electron-trpc/renderer`
+- appRouter lives at `src/main/router/index.ts`; renderer client at `src/renderer/lib/trpc.ts`
+- `@/*` path alias maps to `src/renderer/*` (configured in tsconfig.json paths)
+- All DB tables use `CREATE TABLE IF NOT EXISTS` for non-destructive migrations
+- All IDs are UUIDs generated via `crypto.randomUUID()`
+- TypeScript strict mode + noUncheckedIndexedAccess enabled; always account for possible undefined on array access
+
+---
+
+## 2026-02-19 - US-003
+- What was implemented:
+  - Installed: electron-trpc, @trpc/server, @trpc/client, @trpc/react-query, @tanstack/react-query, zod
+  - Created `src/main/router/index.ts` with full appRouter: stub routers for health, clients, projects, tasks, checkpoints, notes, ai
+  - Updated `src/preload/index.ts` to call `exposeElectronTRPC()`
+  - Updated `src/main/index.ts` to call `createIPCHandler({ router: appRouter, windows: [win] })`; `createWindow()` now returns `BrowserWindow`
+  - Created `src/renderer/lib/trpc.ts` with `createTRPCReact<AppRouter>()`
+  - Updated `src/renderer/index.tsx` to wrap app in `TRPCProvider` + `QueryClientProvider`
+  - Updated `src/renderer/routes/index.tsx` to call `trpc.health.ping.useQuery()` and display 'tRPC IPC bridge: pong'
+- Files changed: package.json, package-lock.json, prd.json, src/main/index.ts, src/main/router/index.ts (new), src/preload/index.ts, src/renderer/index.tsx, src/renderer/lib/trpc.ts (new), src/renderer/routes/index.tsx
+- **Learnings for future iterations:**
+  - electron-trpc `exposeElectronTRPC` is imported from `electron-trpc/main` (not a separate package)
+  - `ipcLink` is imported from `electron-trpc/renderer` in the renderer process
+  - `createTRPCReact<AppRouter>()` requires importing the AppRouter type from the main process router — this is a type-only import so it doesn't bundle main process code into renderer
+  - The TRPCProvider must wrap QueryClientProvider (or be a sibling); both need the same queryClient instance
+  - Stub routers return empty arrays or null — they will be replaced in US-005 through US-008
+---