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/calendar/[userId]/[token].ics/route.test.ts

@@ -4,32 +4,51 @@
 import type { NextRequest } from "next/server";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
-import type { User } from "@/types";
+import type { PeriodLog, User } from "@/types";
 
 // Module-level variable to control mock user lookup
 let mockUsers: Map<string, User> = new Map();
+let mockPeriodLogs: PeriodLog[] = [];
 
 // Mock PocketBase
 vi.mock("@/lib/pocketbase", () => ({
   createPocketBaseClient: vi.fn(() => ({
-    collection: vi.fn(() => ({
-      getOne: vi.fn((userId: string) => {
-        const user = mockUsers.get(userId);
-        if (!user) {
-          const error = new Error("Not found");
-          (error as unknown as { status: number }).status = 404;
-          throw error;
-        }
+    collection: vi.fn((name: string) => {
+      if (name === "users") {
         return {
-          id: user.id,
-          email: user.email,
-          calendarToken: user.calendarToken,
-          lastPeriodDate: user.lastPeriodDate.toISOString(),
-          cycleLength: user.cycleLength,
-          garminConnected: user.garminConnected,
+          getOne: vi.fn((userId: string) => {
+            const user = mockUsers.get(userId);
+            if (!user) {
+              const error = new Error("Not found");
+              (error as unknown as { status: number }).status = 404;
+              throw error;
+            }
+            return {
+              id: user.id,
+              email: user.email,
+              calendarToken: user.calendarToken,
+              lastPeriodDate: user.lastPeriodDate.toISOString(),
+              cycleLength: user.cycleLength,
+              garminConnected: user.garminConnected,
+            };
+          }),
         };
-      }),
-    })),
+      }
+      if (name === "period_logs") {
+        return {
+          getFullList: vi.fn(() =>
+            mockPeriodLogs.map((log) => ({
+              id: log.id,
+              user: log.user,
+              startDate: log.startDate.toISOString(),
+              predictedDate: log.predictedDate?.toISOString() ?? null,
+              created: log.created.toISOString(),
+            })),
+          ),
+        };
+      }
+      return {};
+    }),
   })),
 }));
 
@@ -73,6 +92,7 @@ describe("GET /api/calendar/[userId]/[token].ics", () => {
     vi.clearAllMocks();
     mockUsers = new Map();
     mockUsers.set("user123", mockUser);
+    mockPeriodLogs = [];
   });
 
   // Helper to create route context with params
@@ -228,4 +248,47 @@ describe("GET /api/calendar/[userId]/[token].ics", () => {
 
     expect(response.status).toBe(401);
   });
+
+  it("passes period logs to ICS generator for prediction accuracy", async () => {
+    mockPeriodLogs = [
+      {
+        id: "log1",
+        user: "user123",
+        startDate: new Date("2025-01-10"),
+        predictedDate: new Date("2025-01-12"), // 2 days early
+        created: new Date("2025-01-10"),
+      },
+      {
+        id: "log2",
+        user: "user123",
+        startDate: new Date("2024-12-15"),
+        predictedDate: null, // First log, no prediction
+        created: new Date("2024-12-15"),
+      },
+    ];
+
+    const mockRequest = {} as NextRequest;
+    const context = createRouteContext(
+      "user123",
+      "valid-calendar-token-abc123def",
+    );
+
+    await GET(mockRequest, context);
+
+    expect(mockGenerateIcsFeed).toHaveBeenCalledWith(
+      expect.objectContaining({
+        periodLogs: expect.arrayContaining([
+          expect.objectContaining({
+            id: "log1",
+            startDate: expect.any(Date),
+            predictedDate: expect.any(Date),
+          }),
+          expect.objectContaining({
+            id: "log2",
+            predictedDate: null,
+          }),
+        ]),
+      }),
+    );
+  });
 });