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/cycle.ts

@@ -0,0 +1,73 @@
+// ABOUTME: Cycle phase calculation utilities.
+// ABOUTME: Determines current cycle day and phase from last period date.
+import type { CyclePhase, PhaseConfig } from "@/types";
+
+export const PHASE_CONFIGS: PhaseConfig[] = [
+  {
+    name: "MENSTRUAL",
+    days: [1, 3],
+    weeklyLimit: 30,
+    dailyAvg: 10,
+    trainingType: "Gentle rebounding only",
+  },
+  {
+    name: "FOLLICULAR",
+    days: [4, 14],
+    weeklyLimit: 120,
+    dailyAvg: 17,
+    trainingType: "Strength + rebounding",
+  },
+  {
+    name: "OVULATION",
+    days: [15, 16],
+    weeklyLimit: 80,
+    dailyAvg: 40,
+    trainingType: "Peak performance",
+  },
+  {
+    name: "EARLY_LUTEAL",
+    days: [17, 24],
+    weeklyLimit: 100,
+    dailyAvg: 14,
+    trainingType: "Moderate training",
+  },
+  {
+    name: "LATE_LUTEAL",
+    days: [25, 31],
+    weeklyLimit: 50,
+    dailyAvg: 8,
+    trainingType: "Gentle rebounding ONLY",
+  },
+];
+
+export function getCycleDay(
+  lastPeriodDate: Date,
+  cycleLength: number,
+  currentDate: Date = new Date(),
+): number {
+  const diffMs = currentDate.getTime() - lastPeriodDate.getTime();
+  const diffDays = Math.floor(diffMs / (1000 * 60 * 60 * 24));
+  return (diffDays % cycleLength) + 1;
+}
+
+export function getPhase(cycleDay: number): CyclePhase {
+  for (const config of PHASE_CONFIGS) {
+    if (cycleDay >= config.days[0] && cycleDay <= config.days[1]) {
+      return config.name;
+    }
+  }
+  // Default to late luteal for any days beyond 31
+  return "LATE_LUTEAL";
+}
+
+export function getPhaseConfig(phase: CyclePhase): PhaseConfig {
+  const config = PHASE_CONFIGS.find((c) => c.name === phase);
+  if (!config) {
+    throw new Error(`Unknown phase: ${phase}`);
+  }
+  return config;
+}
+
+export function getPhaseLimit(phase: CyclePhase): number {
+  return getPhaseConfig(phase).weeklyLimit;
+}