feat(automotive): scale the car interface and make it adjustable

A head unit is a large screen sitting an arm's length further away than a phone,
and Plezy drew phone-sized controls on it: the primary button measured 8.3 mm
against the 64 dp a car needs. The whole surface is now scaled - 1.35 by default,
adjustable in Appearance - by giving the app a smaller logical viewport and
scaling the result back, so text, spacing and touch targets grow together instead
of a font size being nudged in isolation.

The scale sits above the messenger and the root Scaffold so snackbars and dialogs
are scaled too, and insets are divided back into the scaled space so a system bar
still reserves its physical size. A scaled surface is also a short one: the setup
screen's fixed offsets and the now-playing transport are laid out to survive it,
and a mistyped scale in a hand-edited settings file is clamped rather than
failing startup.

lib/services/settings_export_service.dart

@@ -121,6 +121,7 @@ class SettingsExportService {
 
     if (pref.key == SettingsService.appLocale.key) return _typeString;
     if (pref.key == SettingsService.libraryDensity.key) return _typeInt;
+    if (pref.key == SettingsService.automotiveUiScale.key) return _typeDouble;
     if (pref.key == SettingsService.autoPip.key ||
         pref.key == SettingsService.useExternalPlayer.key ||
         pref.key == SettingsService.audioPassthrough.key) {

lib/services/settings_service.dart

@@ -135,6 +135,31 @@ class _LibraryDensityPref extends Pref<int> {
       svc.writeInt(key, value.clamp(LibraryDensity.min, LibraryDensity.max));
 }
 
+class AutomotiveUiScale {
+  static const double min = 1.0;
+  static const double max = 2.0;
+  static const double defaultValue = 1.35;
+}
+
+/// Uses a larger default on car displays while honoring and clamping a stored
+/// user adjustment on every platform.
+class _AutomotiveUiScalePref extends Pref<double> {
+  const _AutomotiveUiScalePref() : super('automotive_ui_scale');
+
+  @override
+  double readFrom(BaseSharedPreferencesService svc) {
+    final fallback = PlatformDetector.isAutomotive() ? AutomotiveUiScale.defaultValue : 1.0;
+    // Tolerant read, not `prefs.getDouble`: this is read while building the root
+    // app, so a mistyped stored value would turn every launch into the error
+    // widget instead of dropping the key (#1732).
+    return svc.readDouble(key, defaultValue: fallback).clamp(AutomotiveUiScale.min, AutomotiveUiScale.max).toDouble();
+  }
+
+  @override
+  Future<void> writeTo(BaseSharedPreferencesService svc, double value) =>
+      svc.writeDouble(key, value.clamp(AutomotiveUiScale.min, AutomotiveUiScale.max).toDouble());
+}
+
 /// Migrates from the legacy `use_season_poster` boolean key.
 class _EpisodePosterModePref extends EnumPref<EpisodePosterMode> {
   const _EpisodePosterModePref()
@@ -480,6 +505,7 @@ class SettingsService extends BaseSharedPreferencesService {
 
   static const bufferSize = _BufferSizePref();
   static const libraryDensity = _LibraryDensityPref();
+  static const automotiveUiScale = _AutomotiveUiScalePref();
   static const tvCornerSpotlightBackdrop = BoolPref('tv_corner_spotlight_backdrop');
   static const episodePosterMode = _EpisodePosterModePref();
   static const continueWatchingAction = EnumPref<ContinueWatchingAction>(
@@ -907,6 +933,7 @@ class SettingsService extends BaseSharedPreferencesService {
     videoPlayerNavigationEnabled,
     bufferSize,
     libraryDensity,
+    automotiveUiScale,
     tvCornerSpotlightBackdrop,
     episodePosterMode,
     continueWatchingAction,