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.ts

@@ -17,19 +17,40 @@ interface NotificationResult {
   timestamp: string;
 }
 
-// Get the current hour in a specific timezone
-function getCurrentHourInTimezone(timezone: string): number {
+// Get current quarter-hour slot (0, 15, 30, 45) in user's timezone
+function getCurrentQuarterHourSlot(timezone: string): {
+  hour: number;
+  minute: number;
+} {
   const formatter = new Intl.DateTimeFormat("en-US", {
     timeZone: timezone,
     hour: "numeric",
+    minute: "numeric",
     hour12: false,
   });
-  return parseInt(formatter.format(new Date()), 10);
+  const parts = formatter.formatToParts(new Date());
+  const hour = Number.parseInt(
+    parts.find((p) => p.type === "hour")?.value ?? "0",
+    10,
+  );
+  const minute = Number.parseInt(
+    parts.find((p) => p.type === "minute")?.value ?? "0",
+    10,
+  );
+  // Round down to nearest 15-min slot
+  const slot = Math.floor(minute / 15) * 15;
+  return { hour, minute: slot };
 }
 
-// Extract hour from "HH:MM" format
-function getNotificationHour(notificationTime: string): number {
-  return parseInt(notificationTime.split(":")[0], 10);
+// Extract quarter-hour slot from "HH:MM" format
+function getNotificationSlot(notificationTime: string): {
+  hour: number;
+  minute: number;
+} {
+  const [h, m] = notificationTime.split(":").map(Number);
+  // Round down to nearest 15-min slot
+  const slot = Math.floor(m / 15) * 15;
+  return { hour: h, minute: slot };
 }
 
 // Map decision status to icon
@@ -95,11 +116,14 @@ export async function POST(request: Request) {
 
   for (const user of users) {
     try {
-      // Check if current hour in user's timezone matches their notification time
-      const currentHour = getCurrentHourInTimezone(user.timezone);
-      const notificationHour = getNotificationHour(user.notificationTime);
+      // Check if current quarter-hour slot in user's timezone matches their notification time
+      const currentSlot = getCurrentQuarterHourSlot(user.timezone);
+      const notificationSlot = getNotificationSlot(user.notificationTime);
 
-      if (currentHour !== notificationHour) {
+      if (
+        currentSlot.hour !== notificationSlot.hour ||
+        currentSlot.minute !== notificationSlot.minute
+      ) {
         result.skippedWrongTime++;
         continue;
       }