T15-T18, T20, T23: app shell, three screens, and the first UI tests

Theme, go_router shell, record screen, trips list with selection, trip detail
with stats and charts, rename/delete/merge. 15 widget tests -- the first this
project has ever had, closing the gap docs/v3/BACKLOG.md names as v2's largest.

The theme sets bodyColor and displayColor explicitly rather than relying on a
wrapping widget, so the black-on-black regression cannot recur by someone
removing a container, and a widget test now asserts the headline names a colour
distinct from the ground.

Those tests immediately found a real layout bug: with a ride active the record
screen grows to six stat rows and overflows a short screen. Compose clipped this
silently, so it may be latent in the native app; Flutter reports it. Fixed by
making the screen scrollable while still centring when there is room.

Two harness lessons. pumpAndSettle never settles against a repeating timer, so
the elapsed clock now runs only while a ride is active -- better behaviour
anyway, since an idle screen has no clock to advance. And flutter_test asserts
no Timer is pending after disposal, which Drift's stream-query keep-alive cache
trips; tests now tear the tree down and pump past that window.

160 tests passing, analyze clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/src/app/providers.dart

@@ -16,6 +16,7 @@ import '../domain/models.dart';
 import '../recording/geolocator_location_source.dart';
 import '../recording/location_source.dart';
 import '../recording/recording_engine.dart';
+import '../telemetry/live_telemetry.dart';
 
 /// The Drift database, opened against app-private storage.
 ///
@@ -47,6 +48,10 @@ final recordingEngineProvider = Provider<RecordingEngine>((ref) {
   return engine;
 });
 
+/// The live speedo feed, published from the fix callback at GPS rate.
+final liveTelemetryProvider =
+    Provider<LiveTelemetry>((ref) => LiveTelemetry.instance);
+
 // --- Reactive state ---------------------------------------------------------
 
 /// The in-progress ride, straight from the database.