Implement GET /api/cycle/current endpoint (P1.3)

Add endpoint returning current cycle day, phase, phase configuration,
and days until next phase. Uses withAuth middleware for authentication.

Response shape:
- cycleDay: current day in menstrual cycle (1-31)
- phase: current phase (MENSTRUAL, FOLLICULAR, OVULATION, EARLY_LUTEAL, LATE_LUTEAL)
- phaseConfig: full configuration including weeklyLimit, trainingType
- daysUntilNextPhase: days remaining in current phase
- cycleLength: user's configured cycle length

Includes 10 tests covering:
- Authentication (401 when not authenticated)
- Validation (400 when no lastPeriodDate)
- All five cycle phases
- Cycle rollover handling
- Custom cycle lengths

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

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

src/app/api/cycle/current/route.ts

@@ -2,7 +2,66 @@
 // ABOUTME: Returns current cycle day, phase, and phase limits.
 import { NextResponse } from "next/server";
 
-export async function GET() {
-  // TODO: Implement current cycle info
-  return NextResponse.json({ message: "Not implemented" }, { status: 501 });
+import { withAuth } from "@/lib/auth-middleware";
+import {
+  getCycleDay,
+  getPhase,
+  getPhaseConfig,
+  PHASE_CONFIGS,
+} from "@/lib/cycle";
+
+/**
+ * Calculates the number of days until the next phase begins.
+ * For LATE_LUTEAL, calculates days until new cycle starts (MENSTRUAL).
+ */
+function getDaysUntilNextPhase(cycleDay: number, cycleLength: number): number {
+  const currentPhase = getPhase(cycleDay);
+  const currentConfig = getPhaseConfig(currentPhase);
+
+  // For LATE_LUTEAL, calculate days until new cycle
+  if (currentPhase === "LATE_LUTEAL") {
+    return cycleLength - cycleDay + 1;
+  }
+
+  // Find next phase start day
+  const currentIndex = PHASE_CONFIGS.findIndex((c) => c.name === currentPhase);
+  const nextConfig = PHASE_CONFIGS[currentIndex + 1];
+
+  if (nextConfig) {
+    return nextConfig.days[0] - cycleDay;
+  }
+
+  // Fallback: days until end of current phase + 1
+  return currentConfig.days[1] - cycleDay + 1;
 }
+
+export const GET = withAuth(async (_request, user) => {
+  // Validate user has required cycle data
+  if (!user.lastPeriodDate) {
+    return NextResponse.json(
+      {
+        error:
+          "User has no lastPeriodDate set. Please log your period start date first.",
+      },
+      { status: 400 },
+    );
+  }
+
+  // Calculate current cycle position
+  const cycleDay = getCycleDay(
+    user.lastPeriodDate,
+    user.cycleLength,
+    new Date(),
+  );
+  const phase = getPhase(cycleDay);
+  const phaseConfig = getPhaseConfig(phase);
+  const daysUntilNextPhase = getDaysUntilNextPhase(cycleDay, user.cycleLength);
+
+  return NextResponse.json({
+    cycleDay,
+    phase,
+    phaseConfig,
+    daysUntilNextPhase,
+    cycleLength: user.cycleLength,
+  });
+});