Fix critical bug: cycle phase boundaries now scale with cycle length

CRITICAL BUG FIX:
- Phase boundaries were hardcoded for 31-day cycle, breaking correct
  phase calculations for users with different cycle lengths (28, 35, etc.)
- Added getPhaseBoundaries(cycleLength) function in cycle.ts
- Updated getPhase() to accept cycleLength parameter (default 31)
- Updated all callers (API routes, components) to pass cycleLength
- Added 13 new tests for phase boundaries with 28, 31, and 35-day cycles

ICS IMPROVEMENTS:
- Fixed emojis to match calendar.md spec: 🩸🌱🌸🌙🌑
- Added CATEGORIES field for calendar app colors per spec:
  MENSTRUAL=Red, FOLLICULAR=Green, OVULATION=Pink,
  EARLY_LUTEAL=Yellow, LATE_LUTEAL=Orange
- Added 5 new tests for CATEGORIES

Updated IMPLEMENTATION_PLAN.md with discovered issues and test counts.

825 tests passing (up from 807)

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

src/lib/cycle.ts

@@ -1,7 +1,10 @@
 // ABOUTME: Cycle phase calculation utilities.
-// ABOUTME: Determines current cycle day and phase from last period date.
+// ABOUTME: Determines current cycle day and phase from last period date with variable cycle lengths.
 import type { CyclePhase, PhaseConfig } from "@/types";
 
+// Base phase configurations with weekly limits and training guidance.
+// Note: The 'days' field is for the default 31-day cycle; actual boundaries
+// are calculated dynamically by getPhaseBoundaries() based on cycleLength.
 export const PHASE_CONFIGS: PhaseConfig[] = [
   {
     name: "MENSTRUAL",
@@ -12,21 +15,21 @@ export const PHASE_CONFIGS: PhaseConfig[] = [
   },
   {
     name: "FOLLICULAR",
-    days: [4, 14],
+    days: [4, 15],
     weeklyLimit: 120,
     dailyAvg: 17,
     trainingType: "Strength + rebounding",
   },
   {
     name: "OVULATION",
-    days: [15, 16],
+    days: [16, 17],
     weeklyLimit: 80,
     dailyAvg: 40,
     trainingType: "Peak performance",
   },
   {
     name: "EARLY_LUTEAL",
-    days: [17, 24],
+    days: [18, 24],
     weeklyLimit: 100,
     dailyAvg: 14,
     trainingType: "Moderate training",
@@ -40,6 +43,26 @@ export const PHASE_CONFIGS: PhaseConfig[] = [
   },
 ];
 
+// Phase boundaries scale based on cycle length using fixed luteal, variable follicular.
+// Per spec: luteal phase is biologically consistent (14 days); follicular expands/contracts.
+// Formula from specs/cycle-tracking.md:
+//   MENSTRUAL: 1-3 (fixed)
+//   FOLLICULAR: 4 to (cycleLength - 16)
+//   OVULATION: (cycleLength - 15) to (cycleLength - 14)
+//   EARLY_LUTEAL: (cycleLength - 13) to (cycleLength - 7)
+//   LATE_LUTEAL: (cycleLength - 6) to cycleLength
+function getPhaseBoundaries(
+  cycleLength: number,
+): Array<{ phase: CyclePhase; start: number; end: number }> {
+  return [
+    { phase: "MENSTRUAL", start: 1, end: 3 },
+    { phase: "FOLLICULAR", start: 4, end: cycleLength - 16 },
+    { phase: "OVULATION", start: cycleLength - 15, end: cycleLength - 14 },
+    { phase: "EARLY_LUTEAL", start: cycleLength - 13, end: cycleLength - 7 },
+    { phase: "LATE_LUTEAL", start: cycleLength - 6, end: cycleLength },
+  ];
+}
+
 export function getCycleDay(
   lastPeriodDate: Date,
   cycleLength: number,
@@ -50,13 +73,15 @@ export function getCycleDay(
   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;
+export function getPhase(cycleDay: number, cycleLength = 31): CyclePhase {
+  const boundaries = getPhaseBoundaries(cycleLength);
+
+  for (const { phase, start, end } of boundaries) {
+    if (cycleDay >= start && cycleDay <= end) {
+      return phase;
     }
   }
-  // Default to late luteal for any days beyond 31
+  // Default to late luteal for any days beyond cycle length
   return "LATE_LUTEAL";
 }