Changes
- Added reliable FAT create/modify timestamps for SD-card files, using the live RTC/system clock when available and the last known Sync Day time as a fallback, so screenshots, logs, downloads, caches, and other generated files can be sorted by date (#161).
- Preserved SdFat's standard compile-date placeholder when no valid wall clock is known, and kept the timestamp callback off the RTC I2C path during file writes.