Initial project setup for PhaseFlow

Set up Next.js 16 project with TypeScript for a training decision app
that integrates menstrual cycle phases with Garmin biometrics for
Hashimoto's thyroiditis management.

Stack: Next.js 16, React 19, Tailwind/shadcn, PocketBase, Drizzle,
Zod, Resend, Vitest, Biome, Lefthook, Nix dev environment.

Includes:
- 7 page routes (dashboard, login, settings, calendar, history, plan)
- 12 API endpoints (garmin, user, cycle, calendar, overrides, cron)
- Core lib utilities (decision engine, cycle phases, nutrition, ICS)
- Type definitions and component scaffolding
- Python script for Garmin token bootstrapping
- Initial unit tests for cycle utilities

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

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

src/app/api/cron/garmin-sync/route.ts

@@ -0,0 +1,16 @@
+// ABOUTME: Cron endpoint for syncing Garmin data.
+// ABOUTME: Fetches body battery, HRV, and intensity minutes for all users.
+import { NextResponse } from "next/server";
+
+export async function POST(request: Request) {
+  // Verify cron secret
+  const authHeader = request.headers.get("authorization");
+  const expectedSecret = process.env.CRON_SECRET;
+
+  if (!expectedSecret || authHeader !== `Bearer ${expectedSecret}`) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  // TODO: Implement Garmin data sync
+  return NextResponse.json({ message: "Not implemented" }, { status: 501 });
+}