feat: Implement dynamic subtitle positioning based on menu height

test/util/subtitle_position_calculator_test.dart

@@ -0,0 +1,104 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fladder/models/settings/subtitle_settings_model.dart';
+import 'package:fladder/util/subtitle_position_calculator.dart';
+
+void main() {
+  group('SubtitlePositionCalculator', () {
+    test('returns original offset when overlay is hidden', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.2);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: false,
+        screenHeight: 800,
+        menuHeight: 120,
+      );
+
+      expect(result, equals(0.2));
+    });
+
+    test('uses dynamic menu height when available', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.1);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 800,
+        menuHeight: 120, // 120/800 = 0.15 (15%)
+      );
+
+      // Should position at menu height + dynamic padding = (120/800) + (-0.03) = 0.12
+      expect(result, closeTo(0.12, 0.001));
+    });
+
+    test('uses fallback when menu height unavailable', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.1);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 800,
+        menuHeight: null,
+      );
+
+      // Should use fallback: 0.15 + 0.01 = 0.16
+      expect(result, equals(0.16));
+    });
+
+    test('preserves user offset when already above menu', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.3);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 800,
+        menuHeight: 120,
+      );
+
+      // Should keep original 0.3 since it's above menu area (0.12)
+      expect(result, equals(0.3));
+    });
+
+    test('clamps to maximum offset', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.95);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 800,
+        menuHeight: 600, // Large menu that would push subtitles too high
+      );
+
+      // Should clamp to max 0.85
+      expect(result, equals(0.85));
+    });
+
+    test('handles zero screen height gracefully', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.1);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 0,
+        menuHeight: 120,
+      );
+
+      // Should use fallback when screen height is invalid
+      expect(result, equals(0.16));
+    });
+
+    test('clamps to minimum offset', () {
+      const settings = SubtitleSettingsModel(verticalOffset: 0.05);
+
+      final result = SubtitlePositionCalculator.calculateOffset(
+        settings: settings,
+        showOverlay: true,
+        screenHeight: 800,
+        menuHeight: 120,
+      );
+
+      // Should not go below 0.0
+      expect(result, greaterThanOrEqualTo(0.0));
+    });
+  });
+}