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/lib/garmin.ts

@@ -0,0 +1,39 @@
+// ABOUTME: Garmin Connect API client using stored OAuth tokens.
+// ABOUTME: Fetches body battery, HRV, and intensity minutes from Garmin.
+import type { GarminTokens } from "@/types";
+
+const GARMIN_BASE_URL = "https://connect.garmin.com/modern/proxy";
+
+interface GarminApiOptions {
+  oauth2Token: string;
+}
+
+export async function fetchGarminData(
+  endpoint: string,
+  options: GarminApiOptions,
+): Promise<unknown> {
+  const response = await fetch(`${GARMIN_BASE_URL}${endpoint}`, {
+    headers: {
+      Authorization: `Bearer ${options.oauth2Token}`,
+      NK: "NT",
+    },
+  });
+
+  if (!response.ok) {
+    throw new Error(`Garmin API error: ${response.status}`);
+  }
+
+  return response.json();
+}
+
+export function isTokenExpired(tokens: GarminTokens): boolean {
+  const expiresAt = new Date(tokens.expires_at);
+  return expiresAt <= new Date();
+}
+
+export function daysUntilExpiry(tokens: GarminTokens): number {
+  const expiresAt = new Date(tokens.expires_at);
+  const now = new Date();
+  const diffMs = expiresAt.getTime() - now.getTime();
+  return Math.floor(diffMs / (1000 * 60 * 60 * 24));
+}