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/mode_selector.dart

@@ -0,0 +1,98 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../../theme/autopilot_theme.dart';
+import '../../theme/theme_provider.dart';
+
+/// Autopilot mode selector — STANDBY / HEADING_HOLD / TRACK_KEEP.
+///
+/// The active mode is highlighted with [AutopilotTheme.accentMid].
+/// Inactive modes use muted text and panel border.
+/// Minimum touch target: 48×48 px per mode button.
+enum AutopilotMode { standby, headingHold, trackKeep }
+
+extension AutopilotModeLabel on AutopilotMode {
+  String get label => switch (this) {
+        AutopilotMode.standby => 'STANDBY',
+        AutopilotMode.headingHold => 'HDG HOLD',
+        AutopilotMode.trackKeep => 'TRACK',
+      };
+}
+
+class ModeSelector extends StatelessWidget {
+  const ModeSelector({
+    super.key,
+    required this.activeMode,
+    required this.onModeSelected,
+  });
+
+  final AutopilotMode activeMode;
+  final ValueChanged<AutopilotMode> onModeSelected;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.watch<AutopilotThemeProvider>().current;
+    return Container(
+      decoration: BoxDecoration(
+        gradient: theme.panelBackground,
+        borderRadius: BorderRadius.circular(8),
+        border: Border.all(color: theme.panelBorder),
+      ),
+      child: Row(
+        children: AutopilotMode.values.map((mode) {
+          final isActive = mode == activeMode;
+          return Expanded(
+            child: _ModeButton(
+              theme: theme,
+              mode: mode,
+              isActive: isActive,
+              onTap: () => onModeSelected(mode),
+            ),
+          );
+        }).toList(),
+      ),
+    );
+  }
+}
+
+class _ModeButton extends StatelessWidget {
+  const _ModeButton({
+    required this.theme,
+    required this.mode,
+    required this.isActive,
+    required this.onTap,
+  });
+
+  final AutopilotTheme theme;
+  final AutopilotMode mode;
+  final bool isActive;
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTap: onTap,
+      child: AnimatedContainer(
+        duration: const Duration(milliseconds: 200),
+        constraints: const BoxConstraints(minHeight: 48),
+        alignment: Alignment.center,
+        decoration: BoxDecoration(
+          color: isActive ? theme.accentMid.withValues(alpha: 0.15) : Colors.transparent,
+          borderRadius: BorderRadius.circular(6),
+        ),
+        child: Text(
+          mode.label,
+          style: TextStyle(
+            color: isActive ? theme.accentLight : theme.textMuted,
+            fontSize: 12,
+            fontWeight: isActive ? FontWeight.w700 : FontWeight.w400,
+            letterSpacing: 0.8,
+            shadows: isActive && theme.accentGlowRadius > 0
+                ? [Shadow(color: theme.accentGlowColor, blurRadius: 6)]
+                : null,
+          ),
+        ),
+      ),
+    );
+  }
+}