refactor: remove waveform color setting to preserve dynamic coloring

- Removed waveformColor from UISettings interface
- Removed waveform color picker from Interface settings tab
- Preserves dynamic per-track waveform coloring system
- Cleaner settings UI with one less option

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

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

lib/hooks/useSettings.ts

@@ -10,7 +10,6 @@ export interface AudioSettings {
 
 export interface UISettings {
   theme: 'dark' | 'light' | 'auto';
-  waveformColor: string;
   fontSize: 'small' | 'medium' | 'large';
   defaultTrackHeight: number; // 120-400px
 }
@@ -45,7 +44,6 @@ const DEFAULT_SETTINGS: Settings = {
   },
   ui: {
     theme: 'dark',
-    waveformColor: '#3b82f6', // blue-500
     fontSize: 'medium',
     defaultTrackHeight: 400,
   },