Implement GET /api/user endpoint (P0.4)

Add authenticated user profile retrieval endpoint using withAuth wrapper.
Returns user profile with safe fields, excluding encrypted tokens.

Changes:
- Implement GET handler in src/app/api/user/route.ts
- Add 4 tests for auth, response shape, sensitive field exclusion
- Add path alias resolution to vitest.config.ts for @/* imports
- Update IMPLEMENTATION_PLAN.md to mark P0.4 complete

Response includes: id, email, garminConnected, cycleLength,
lastPeriodDate, notificationTime, timezone, activeOverrides

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

vitest.config.ts

@@ -1,10 +1,16 @@
 // ABOUTME: Vitest configuration for unit and integration testing.
 // ABOUTME: Configures jsdom environment for React component testing.
+import path from "node:path";
 import react from "@vitejs/plugin-react";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
   plugins: [react()],
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
   test: {
     environment: "jsdom",
     include: ["src/**/*.test.{ts,tsx}"],