refactor: remove configurable track height setting

Removed the defaultTrackHeight setting from UI preferences as it doesn't need
to be user-configurable. All tracks now use DEFAULT_TRACK_HEIGHT constant (400px).

Changes:
- Removed defaultTrackHeight from UISettings interface
- Removed track height slider from GlobalSettingsDialog
- Updated AudioEditor to use DEFAULT_TRACK_HEIGHT constant directly
- Simplified dependency arrays in addTrack callbacks

This simplifies the settings interface while maintaining the same visual behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/hooks/useSettings.ts

@@ -11,7 +11,6 @@ export interface AudioSettings {
 export interface UISettings {
   theme: 'dark' | 'light' | 'auto';
   fontSize: 'small' | 'medium' | 'large';
-  defaultTrackHeight: number; // 120-400px
 }
 
 export interface EditorSettings {
@@ -45,7 +44,6 @@ const DEFAULT_SETTINGS: Settings = {
   ui: {
     theme: 'dark',
     fontSize: 'medium',
-    defaultTrackHeight: 400,
   },
   editor: {
     autoSaveInterval: 3, // 3 seconds