Fix email timing and show fallback data when Garmin sync pending

- Add 15-minute notification granularity (*/15 cron) so users get emails
  at their configured time instead of rounding to the nearest hour
- Add DailyLog fallback to most recent when today's log doesn't exist,
  preventing 100/100/Unknown default values before morning sync
- Show "Last synced" indicator when displaying stale data
- Change Garmin sync to 6-hour intervals (0,6,12,18 UTC) to ensure
  data is available before European morning notifications

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

src/app/api/cron/notifications/route.test.ts

@@ -205,6 +205,112 @@ describe("POST /api/cron/notifications", () => {
     });
   });
 
+  describe("Quarter-hour time matching", () => {
+    it("sends notification at exact 15-minute slot (07:15)", async () => {
+      // Current time is 07:15 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:15:00Z"));
+      mockUsers = [
+        createMockUser({ notificationTime: "07:15", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+
+    it("rounds down notification time to nearest 15-minute slot (07:10 -> 07:00)", async () => {
+      // Current time is 07:00 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:00:00Z"));
+      // User set 07:10, which rounds down to 07:00 slot
+      mockUsers = [
+        createMockUser({ notificationTime: "07:10", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+
+    it("rounds down notification time (07:29 -> 07:15)", async () => {
+      // Current time is 07:15 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:15:00Z"));
+      // User set 07:29, which rounds down to 07:15 slot
+      mockUsers = [
+        createMockUser({ notificationTime: "07:29", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+
+    it("does not send notification when minute slot does not match", async () => {
+      // Current time is 07:00 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:00:00Z"));
+      // User wants 07:15, but current slot is 07:00
+      mockUsers = [
+        createMockUser({ notificationTime: "07:15", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).not.toHaveBeenCalled();
+    });
+
+    it("handles 30-minute slot correctly", async () => {
+      // Current time is 07:30 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:30:00Z"));
+      mockUsers = [
+        createMockUser({ notificationTime: "07:30", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+
+    it("handles 45-minute slot correctly", async () => {
+      // Current time is 07:45 UTC
+      vi.setSystemTime(new Date("2025-01-15T07:45:00Z"));
+      mockUsers = [
+        createMockUser({ notificationTime: "07:45", timezone: "UTC" }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+
+    it("handles timezone with 15-minute matching", async () => {
+      // Current time is 07:15 UTC = 02:15 America/New_York (EST is UTC-5)
+      vi.setSystemTime(new Date("2025-01-15T07:15:00Z"));
+      mockUsers = [
+        createMockUser({
+          notificationTime: "02:15",
+          timezone: "America/New_York",
+        }),
+      ];
+      mockDailyLogs = [createMockDailyLog()];
+
+      const response = await POST(createMockRequest(`Bearer ${validSecret}`));
+
+      expect(response.status).toBe(200);
+      expect(mockSendDailyEmail).toHaveBeenCalled();
+    });
+  });
+
   describe("DailyLog handling", () => {
     it("does not send notification if no DailyLog exists for today", async () => {
       mockUsers = [