sprint-4b: Flutter display theme system (AutopilotTheme + 4 palettes + tests)

Implements the visual theme system specified in mock up software.pdf.

Flutter project — display/:
- pubspec.yaml: ar_autopilot_display v0.4.0+4 (provider + shared_preferences)
- lib/theme/autopilot_theme.dart: AutopilotTheme class with 30+ design tokens
  (backgrounds, panels, text, accent, set-point, semantic states, DISENGAGE,
  action buttons, glow helpers, backgroundDecoration getter)
- lib/theme/theme_registry.dart: ThemeRegistry with 4 factory themes,
  byId() fallback, architecture stub for Sprint 9 custom YAML themes
- lib/theme/theme_provider.dart: AutopilotThemeProvider (ChangeNotifier),
  SharedPreferences persistence under 'autopilot.theme.id', NOT sent to ESP32
- lib/theme/themes/: 4 factory themes with exact hex values from spec:
    light (cream/navy, accentGlowRadius=0 — daytime no-glow rule)
    cyan (deep navy/neon-cyan, default, glowRadius=16)
    wine (vinotinto, DISENGAGE=amber not red, glowRadius=18)
    ochre (warm brown/gold, okColor=lime for contrast, glowRadius=18)
- lib/screens/settings/appearance_settings.dart: Appearance screen with
  4-card theme previews (200x120px), 400ms AnimatedContainer transitions,
  triple-tap shortcut note, Sprint-5 placeholders for auto day/night and
  ambient light sensor toggles
- lib/widgets/themed/: 4 themed widgets consuming AutopilotThemeProvider:
    compass_rose.dart (heading arc, N mark, set-point tick, glow ring)
    disengage_button.dart (60x60 min touch target, gradient, glow)
    mode_selector.dart (STANDBY/HDG HOLD/TRACK with accent highlight)
    rudder_indicator.dart (horizontal bar -35° to +35°, accent knob)
- lib/main.dart: app entry point with ChangeNotifierProvider

Tests — display/test/theme/:
- theme_registry_test.dart: 4 themes load, correct IDs, display order,
  no null tokens, glow rules, backgroundGradient rules
- theme_provider_test.dart: default load=cyan, persistence across restarts,
  setTheme notifies listeners, unknown ID falls back to default
- theme_contrast_test.dart: WCAG checks — DISENGAGE ≥7:1 AAA,
  action buttons ≥4.5:1 AA, textMain ≥4.5:1 AA, setLight/okColor ≥3:1

Also:
- .gitignore: added !display/lib/ exception (lib/ was excluded for Python venv)

Design rules enforced:
- No glow in light mode (accentGlowRadius=0)
- DISENGAGE = amber in wine theme (red would blend into palette)
- North mark = warm colour on all themes (nautical convention)
- Touch targets: 48px nominal, 60px critical
- Theme not sent to ESP32, not synced between displays

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

display/lib/widgets/themed/disengage_button.dart

@@ -0,0 +1,107 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../../theme/autopilot_theme.dart';
+import '../../theme/theme_provider.dart';
+
+/// The DISENGAGE emergency button.
+///
+/// Always the highest-contrast element on screen.
+/// Minimum touch target: 60×60 px (critical control).
+///
+/// The gradient and glow colours adapt to the active theme automatically.
+/// In the wine theme the button is amber-gold (not red) — see [wineTheme].
+class DisengageButton extends StatelessWidget {
+  const DisengageButton({
+    super.key,
+    required this.onPressed,
+    this.enabled = true,
+  });
+
+  final VoidCallback? onPressed;
+  final bool enabled;
+
+  /// Minimum touch target for a critical control.
+  static const double kMinSize = 60.0;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.watch<AutopilotThemeProvider>().current;
+    return _DisengageButtonContent(
+      theme: theme,
+      onPressed: enabled ? onPressed : null,
+    );
+  }
+}
+
+class _DisengageButtonContent extends StatefulWidget {
+  const _DisengageButtonContent({
+    required this.theme,
+    required this.onPressed,
+  });
+
+  final AutopilotTheme theme;
+  final VoidCallback? onPressed;
+
+  @override
+  State<_DisengageButtonContent> createState() =>
+      _DisengageButtonContentState();
+}
+
+class _DisengageButtonContentState
+    extends State<_DisengageButtonContent> {
+  bool _pressed = false;
+
+  @override
+  Widget build(BuildContext context) {
+    final t = widget.theme;
+    final enabled = widget.onPressed != null;
+
+    return GestureDetector(
+      onTapDown: enabled ? (_) => setState(() => _pressed = true) : null,
+      onTapUp: enabled
+          ? (_) {
+              setState(() => _pressed = false);
+              widget.onPressed!();
+            }
+          : null,
+      onTapCancel: () => setState(() => _pressed = false),
+      child: AnimatedContainer(
+        duration: const Duration(milliseconds: 120),
+        constraints: const BoxConstraints(
+          minWidth: DisengageButton.kMinSize,
+          minHeight: DisengageButton.kMinSize,
+        ),
+        padding: const EdgeInsets.symmetric(horizontal: 20, vertical: 14),
+        decoration: BoxDecoration(
+          gradient: t.disengageBackground,
+          borderRadius: BorderRadius.circular(8),
+          border: Border.all(color: t.disengageBorder, width: 1.5),
+          boxShadow: [
+            BoxShadow(
+              color: t.disengageGlow,
+              blurRadius: _pressed ? 4 : 12,
+              spreadRadius: _pressed ? 0 : 2,
+            ),
+          ],
+        ),
+        child: Text(
+          'DISENGAGE',
+          textAlign: TextAlign.center,
+          style: TextStyle(
+            color: t.disengageText,
+            fontSize: 13,
+            fontWeight: FontWeight.w800,
+            letterSpacing: 1.2,
+            shadows: [
+              Shadow(
+                color: t.disengageGlow,
+                blurRadius: 4,
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}