M2 step 6: Tracks, recorded to GPX in the background

`r` in the GNSS App (or `gnss track start|stop`) records a Track to
/gnss/tracks/YYYYMMDD-HHMMSS.gpx: a point every 5 s once moved 5 m
(lib/gnss/track, 7 tests: haversine, the rule, GPX text, the file name).
It keeps recording with the App closed, shows REC in the Status Bar, and
announces start and stop with a Toast. It needs a card and the time;
switching GNSS off stops it. Tracks are written like Captures: past 90 %
card usage, until full. Storage Clean-up gets a GNSS tracks category.

A Track cut short by a reset or power loss has no GPX footer; the GNSS
Service closes such files at the next boot.

Verified on the device: a recorded Track and one interrupted by a reset
both parse as GPX 1.1 on the PC.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/storage_model/src/cleanup_plan.h

@@ -20,6 +20,7 @@ inline const CleanupCategory kCleanupCategories[] = {
     {"IRC logs", "/irc"},
     {"Wi-Fi scan logs", "/wifi/scans"},
     {"Wi-Fi captures", "/captures/wifi"},
+    {"GNSS tracks", "/gnss/tracks"},
 };
 
 enum class CleanupAge { OneMonth, ThreeMonths, SixMonths, OneYear, Everything };