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/app/api/cycle/period/route.ts

@@ -89,7 +89,7 @@ export const POST = withAuth(async (request: NextRequest, user) => {
     // Calculate updated cycle information
     const lastPeriodDate = new Date(body.startDate);
     const cycleDay = getCycleDay(lastPeriodDate, user.cycleLength, new Date());
-    const phase = getPhase(cycleDay);
+    const phase = getPhase(cycleDay, user.cycleLength);
 
     // Calculate prediction accuracy
     let daysEarly: number | undefined;