UI-01: persistent 4-tab shell with a shared background map

Replaces the push/pop stack rooted at Record with a StatefulShellRoute
(Map/Rides/Plan/Settings), each tab keeping its own navigator so Trip Detail
and the route planner push within their own branch. AppShell/ShellScaffold
host one shared RideMap instance behind every tab -- full opacity and
interactive on Map, dimmed and non-interactive elsewhere -- so the camera
position and live path survive a tab switch instead of being refetched.
Every tab's AppBar/header is removed per the redesign's no-chrome mandate.

Fixes a bug this surfaced: RideMap rendered a structurally different tree
for empty vs. non-empty points, which crashed once the map became a
long-lived shell background instead of a fresh per-screen widget. Unified
the background-map tree shape so it no longer remounts mid-recording.

Verified on Medium_Phone_API_35: all four tabs, live recording surviving
tab switches with the camera preserved, and the real HOME-key lifecycle
tile-drop from V3-04 still firing correctly in the shell context.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

lib/src/ui/trips/trips_screen.dart

@@ -21,10 +21,9 @@ final _selectionProvider = StateProvider.autoDispose<Set<int>>(
 );
 
 class TripsScreen extends ConsumerWidget {
-  const TripsScreen({super.key, this.onOpenTrip, this.onBack});
+  const TripsScreen({super.key, this.onOpenTrip});
 
   final void Function(int tripId)? onOpenTrip;
-  final VoidCallback? onBack;
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
@@ -44,18 +43,22 @@ class TripsScreen extends ConsumerWidget {
     void clearSelection() =>
         ref.read(_selectionProvider.notifier).state = <int>{};
 
+    // UI-01: no back/title header -- Rides is a tab now. The selection toolbar (Cancel/
+    // count/Merge/Delete) is functional, not chrome, so it stays -- it only appears
+    // while `selecting` is true.
     return Scaffold(
+      backgroundColor: Colors.transparent,
       body: SafeArea(
         child: Padding(
           padding: const EdgeInsets.symmetric(horizontal: 16),
           child: Column(
             crossAxisAlignment: CrossAxisAlignment.stretch,
             children: [
-              Padding(
-                padding: const EdgeInsets.symmetric(vertical: 12),
-                child: Row(
-                  children: [
-                    if (selecting) ...[
+              if (selecting)
+                Padding(
+                  padding: const EdgeInsets.symmetric(vertical: 12),
+                  child: Row(
+                    children: [
                       TextButton(
                         onPressed: clearSelection,
                         child: const Text('Cancel'),
@@ -67,27 +70,7 @@ class TripsScreen extends ConsumerWidget {
                           style: TextStyle(fontSize: 14, color: colors.outline),
                         ),
                       ),
-                    ] else ...[
-                      TextButton(
-                        key: const Key('back'),
-                        onPressed: onBack,
-                        child: const Text('‹ Record'),
-                      ),
-                      Padding(
-                        padding: const EdgeInsets.only(left: 8),
-                        child: Text(
-                          'RIDES',
-                          style: TextStyle(
-                            fontSize: 18,
-                            fontWeight: FontWeight.bold,
-                            letterSpacing: 3,
-                            color: colors.primary,
-                          ),
-                        ),
-                      ),
-                    ],
-                    const Spacer(),
-                    if (selecting) ...[
+                      const Spacer(),
                       TextButton(
                         key: const Key('merge'),
                         onPressed: canMerge
@@ -104,9 +87,8 @@ class TripsScreen extends ConsumerWidget {
                         ),
                       ),
                     ],
-                  ],
+                  ),
                 ),
-              ),
               Expanded(
                 child: switch (tripsAsync) {
                   AsyncValue(hasValue: true, value: final list)