docs: note system back handling in trip replay

PLAN.md

@@ -2209,7 +2209,7 @@ The app reuses the existing Pi REST API surface with no changes:
 - `TripListScreen` (new Trips tab): date/time, distance, duration, avg/max speed, start→end labels.
 - `TripReplayScreen`: MapLibre map using the Pi's offline raster tiles (`/static/vendor/tiles/{z}/{x}/{y}.png`,
   zoom 8–15 — no internet needed), route polyline + cursor marker, camera fit, timeline scrubber
-  with timestamp + speed, and play/pause animation.
+  with timestamp + speed, and play/pause animation. System back returns to the list instead of exiting.
 - Deferred: Room offline cache, start/end markers, geocode label under the scrubber.
 
 **28.6 — Remote portal direct access ✅ DONE (2026-08-15)**

android/plan.md

@@ -201,6 +201,7 @@ android/
 - [x] Timeline scrubber (Slider) with timestamp + speed label
 - [x] Play/pause button animating the cursor along the route (~5 fps)
 - [x] Trip stats header: distance, duration, avg speed, max speed
+- [x] System back button returns from replay to the list (BackHandler)
 - [ ] Start/end markers + geocode label below scrubber — deferred
 
 ### 28.6 — Remote portal direct access ✅ DONE (2026-08-15)