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/instrumentation.ts

@@ -43,20 +43,21 @@ export async function register() {
       }
     }
 
-    // Schedule notifications at the top of every hour
-    cron.default.schedule("0 * * * *", () => {
+    // Schedule notifications every 15 minutes for finer-grained delivery times
+    cron.default.schedule("*/15 * * * *", () => {
       console.log("[cron] Triggering notifications...");
       triggerCronEndpoint("notifications", "Notifications");
     });
 
-    // Schedule Garmin sync 3 times daily (8 AM, 2 PM, 10 PM UTC)
-    cron.default.schedule("0 8,14,22 * * *", () => {
+    // Schedule Garmin sync 4 times daily (every 6 hours) to ensure data is available
+    // before European morning notifications
+    cron.default.schedule("0 0,6,12,18 * * *", () => {
       console.log("[cron] Triggering Garmin sync...");
       triggerCronEndpoint("garmin-sync", "Garmin sync");
     });
 
     console.log(
-      "[cron] Scheduler started - notifications hourly, Garmin sync at 08:00/14:00/22:00 UTC",
+      "[cron] Scheduler started - notifications every 15 min, Garmin sync at 00:00/06:00/12:00/18:00 UTC",
     );
   }
 }