Add period prediction accuracy feedback (P4.5 complete)

Implements visual feedback for cycle prediction accuracy in ICS calendar feeds:

- Add predictedDate field to PeriodLog type for tracking predicted vs actual dates
- POST /api/cycle/period now calculates and stores predictedDate based on
  previous lastPeriodDate + cycleLength, returns daysEarly/daysLate in response
- ICS feed generates "(Predicted)" events when actual period start differs
  from predicted, with descriptions like "period arrived 2 days early"
- Calendar route fetches period logs and passes them to ICS generator

This creates an accuracy feedback loop helping users understand their cycle
variability over time per calendar.md spec.

807 tests passing across 43 test files.

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

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

@@ -65,15 +65,25 @@ export const POST = withAuth(async (request: NextRequest, user) => {
 
     const pb = createPocketBaseClient();
 
+    // Calculate predicted date based on previous cycle (if exists)
+    let predictedDateStr: string | null = null;
+    if (user.lastPeriodDate) {
+      const previousPeriod = new Date(user.lastPeriodDate);
+      const predictedDate = new Date(previousPeriod);
+      predictedDate.setDate(previousPeriod.getDate() + user.cycleLength);
+      predictedDateStr = predictedDate.toISOString().split("T")[0];
+    }
+
     // Update user's lastPeriodDate
     await pb.collection("users").update(user.id, {
       lastPeriodDate: body.startDate,
     });
 
-    // Create PeriodLog record
+    // Create PeriodLog record with prediction data
     await pb.collection("period_logs").create({
       user: user.id,
       startDate: body.startDate,
+      predictedDate: predictedDateStr,
     });
 
     // Calculate updated cycle information
@@ -81,6 +91,22 @@ export const POST = withAuth(async (request: NextRequest, user) => {
     const cycleDay = getCycleDay(lastPeriodDate, user.cycleLength, new Date());
     const phase = getPhase(cycleDay);
 
+    // Calculate prediction accuracy
+    let daysEarly: number | undefined;
+    let daysLate: number | undefined;
+    if (predictedDateStr) {
+      const actual = new Date(body.startDate);
+      const predicted = new Date(predictedDateStr);
+      const diffDays = Math.floor(
+        (predicted.getTime() - actual.getTime()) / (1000 * 60 * 60 * 24),
+      );
+      if (diffDays > 0) {
+        daysEarly = diffDays;
+      } else if (diffDays < 0) {
+        daysLate = Math.abs(diffDays);
+      }
+    }
+
     // Log successful period logging per observability spec
     logger.info({ userId: user.id, date: body.startDate }, "Period logged");
 
@@ -89,6 +115,9 @@ export const POST = withAuth(async (request: NextRequest, user) => {
       lastPeriodDate: body.startDate,
       cycleDay,
       phase,
+      ...(predictedDateStr && { predictedDate: predictedDateStr }),
+      ...(daysEarly !== undefined && { daysEarly }),
+      ...(daysLate !== undefined && { daysLate }),
     });
   } catch (error) {
     logger.error({ err: error, userId: user.id }, "Period logging error");