Fix Garmin sync to handle PocketBase date strings

PocketBase returns date fields as ISO strings, not Date objects.
The sync was failing with "e.getTime is not a function" because
the code expected Date objects.

- Export mapRecordToUser from pocketbase.ts
- Use mapRecordToUser in cron route to properly parse dates
- Add test for handling date fields as ISO strings

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

src/lib/pocketbase.ts

@@ -88,7 +88,7 @@ function parseDate(value: unknown): Date | null {
 /**
  * Maps a PocketBase record to our typed User interface.
  */
-function mapRecordToUser(record: RecordModel): User {
+export function mapRecordToUser(record: RecordModel): User {
   return {
     id: record.id,
     email: record.email as string,