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/widgets/fitting_title_text.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import '../utils/platform_detector.dart';
 
 class FittingTitleText extends StatelessWidget {
   final String text;
@@ -23,6 +24,12 @@ class FittingTitleText extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final baseStyle = style ?? DefaultTextStyle.of(context).style;
+    if (PlatformDetector.isAutomotive()) {
+      return Align(
+        alignment: alignment,
+        child: Text(text, style: baseStyle, maxLines: maxLines, overflow: overflow, textAlign: textAlign),
+      );
+    }
     return LayoutBuilder(
       builder: (context, constraints) {
         var fittedStyle = baseStyle;