feat: Implement dynamic subtitle positioning based on menu height

lib/util/subtitle_position_calculator.dart

@@ -0,0 +1,65 @@
+import 'dart:math' as math;
+import 'package:fladder/models/settings/subtitle_settings_model.dart';
+
+/// Utility class for calculating subtitle positioning based on menu overlay state
+/// Provides utilities for calculating the optimal vertical position of subtitles
+/// based on user settings and the visibility or size of the player menu overlay.
+class SubtitlePositionCalculator {
+  // Configuration constants
+  static const double _fallbackMenuHeightPercentage = 0.15; // 15% fallback
+  static const double _dynamicSubtitlePadding =
+      -0.03; // -3% padding when we have accurate menu height so the subtitles are closer to the menu
+  static const double _fallbackSubtitlePadding = 0.01; // 1% padding for conservative fallback positioning
+  static const double _maxSubtitleOffset = 0.85; // Max 85% up from bottom
+
+  /// Calculate subtitle offset using actual menu height when available
+  ///
+  /// Returns the optimal subtitle offset (0.0 to 1.0) where:
+  /// - 0.0 = bottom of screen
+  /// - 1.0 = top of screen
+  ///
+  /// Parameters:
+  /// - [settings]: User's subtitle settings containing preferred vertical offset
+  /// - [showOverlay]: Whether the player menu overlay is currently visible
+  /// - [screenHeight]: Height of the screen in pixels
+  /// - [menuHeight]: Optional actual height of the menu in pixels
+  static double calculateOffset({
+    required SubtitleSettingsModel settings,
+    required bool showOverlay,
+    required double screenHeight,
+    double? menuHeight,
+  }) {
+    if (!showOverlay) {
+      return settings.verticalOffset;
+    }
+
+    double menuHeightPercentage;
+    double subtitlePadding;
+
+    if (menuHeight != null && screenHeight > 0) {
+      // Convert menu height from pixels to screen percentage
+      menuHeightPercentage = menuHeight / screenHeight;
+      // Use negative padding since we have accurate measurement - can position closer
+      subtitlePadding = _dynamicSubtitlePadding;
+    } else {
+      // Fallback to static percentage when measurement unavailable
+      menuHeightPercentage = _fallbackMenuHeightPercentage;
+      // Use positive padding for safety since we're estimating
+      subtitlePadding = _fallbackSubtitlePadding;
+    }
+
+    // Calculate the minimum safe position (menu height + appropriate padding)
+    final minSafeOffset = menuHeightPercentage + subtitlePadding;
+
+    // If subtitles are already positioned above the safe area, leave them alone
+    // but still apply maximum bounds checking
+    if (settings.verticalOffset >= minSafeOffset) {
+      return math.min(settings.verticalOffset, _maxSubtitleOffset);
+    }
+
+    // Position subtitles just above the menu with bounds checking
+    // Defensive: math.max(0.0, ...) ensures the offset is never negative,
+    // which could happen if future changes allow negative menuHeight or padding.
+    return math.max(0.0, math.min(minSafeOffset, _maxSubtitleOffset));
+  }
+}