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/theme/themes/light_theme.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+import '../autopilot_theme.dart';
+
+/// **light** — Claro (día)
+///
+/// Daytime operation under direct sunlight. Navy-blue accent on cream white.
+/// No glow effects — bloom is invisible and distracting in bright ambient light.
+const AutopilotTheme lightTheme = AutopilotTheme(
+  id: 'light',
+  displayName: 'Claro (día)',
+
+  background: Color(0xFFF5F4EE),
+  backgroundMid: Color(0xFFFFFFFF),
+  backgroundDeep: Color(0xFFF1EFE8),
+  backgroundDeepest: Color(0xFFE8E6DD),
+  backgroundGradient: null, // solid background — no gradient in light mode
+
+  panelBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xFFFFFFFF), Color(0xFFFFFFFF)],
+  ),
+  panelBorder: Color(0x1F000000), // rgba(0,0,0,0.12)
+
+  textMain: Color(0xFF2C2C2A),
+  textMuted: Color(0xFF6B6862),
+  textSoft: Color(0xFF2C2C2A),
+  textDisabled: Color(0xFFB4B2A9),
+
+  accentLight: Color(0xFF378ADD),
+  accentMid: Color(0xFF185FA5),
+  accentDark: Color(0xFF0C447C),
+  accentGlowRadius: 0.0, // no glow in daytime mode
+  accentGlowColor: Colors.transparent,
+
+  setLight: Color(0xFFCA8A04),
+  setDark: Color(0xFF854F0B),
+  setGlow: Color(0x33CA8A04),
+
+  okColor: Color(0xFF15803D),
+  warnColor: Color(0xFFCA8A04),
+  northColor: Color(0xFFB91C1C),
+
+  disengageBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xFFDC2626), Color(0xFF991B1B)],
+  ),
+  disengageText: Color(0xFFFFFFFF),
+  disengageBorder: Color(0xFFEF4444),
+  disengageGlow: Color(0x4DDC2626),
+
+  actionButtonBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xFFFFFFFF), Color(0xFFF1EFE8)],
+  ),
+  actionButtonBorder: Color(0x33000000), // 0.2 alpha — more visible than 0.12
+  actionButtonText: Color(0xFF2C2C2A),
+  actionButtonGlow: Colors.transparent,
+);