GPX export + GPS background recording fixes

GPX export: button in session detail, standard GPX 1.1 format.

Background GPS fixes:
- Native service: batched writes (10s flush) instead of per-point
  appendText — eliminates race condition with reader
- JS reader: skip corrupt lines, sort by timestamp, track by ts
  instead of line count — handles partial writes and reordering
- Recovery: idempotent import using max DB timestamp
- Live recording: seq numbers on every point
- FileObserver watches correct path (app root, not files/dev.baldav.app)
- Removed ACCESS_BACKGROUND_LOCATION (causes throttling)

Nav: "Трекинг" button on GPS screen when recording active.

frontend/css/styles.css

@@ -1158,6 +1158,7 @@ body {
   border-radius: var(--radius);
   overflow: hidden;
   cursor: pointer;
+  touch-action: pan-y pinch-zoom;
 }
 
 .sd-map-full {