FB-01: ambient GPS position drives the shared background map while idle

Adds ambientPositionProvider (never calls LocationSource.stop(), only the
idempotent start()) and wires it into ShellScaffold whenever no trip is
active, so the persistent background map centers and follows the device's
live GPS position at street-level zoom instead of sitting at (0,0)/zoom 2
until a recording starts. RideMap gains an ambientPosition param that the
existing chase-camera/pan-cancel/location-marker logic falls back to
whenever there are no recorded points, with recorded points always taking
priority. Also brings in the docs/feedback ticket set (FB-01..FB-05, README,
FEEDBACK.md) that this worktree's branch point predated.

Adds 10 tests (374 -> 384): RideMap-level ambient centering/chase/pan-cancel/
marker coverage in test/ride_map_test.dart, plus shell-wiring and
provider-level (never-calls-stop, mapEnabledProvider-off) coverage in
test/widget_test.dart. flutter analyze remains clean.

lib/src/ui/app_shell.dart

@@ -14,6 +14,7 @@ library;
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import 'package:latlong2/latlong.dart' as ll;
 
 import '../app/providers.dart';
 import '../domain/models.dart';
@@ -70,6 +71,13 @@ class ShellScaffold extends ConsumerWidget {
     final segments = trip == null
         ? const <Segment>[]
         : ref.watch(liveSegmentsProvider(trip.id)).valueOrNull ?? const <Segment>[];
+    // FB-01: only watched while idle, so there's never a duplicate GPS consumer during
+    // an active recording -- the moment a trip starts, this stops being watched at all.
+    final ambientFix =
+        trip == null ? ref.watch(ambientPositionProvider).valueOrNull : null;
+    final ambientPosition = ambientFix == null
+        ? null
+        : ll.LatLng(ambientFix.latitude, ambientFix.longitude);
 
     return Scaffold(
       // Falls back to the ordinary theme background when the map is disabled -- the
@@ -90,6 +98,7 @@ class ShellScaffold extends ConsumerWidget {
                   key: const Key('shell-background-map'),
                   points: points,
                   segments: segments,
+                  ambientPosition: ambientPosition,
                   follow: isMapTab,
                   fill: true,
                   showEmptyLabel: false,