fix: migrate existing tracks to new 180px height from localStorage

Added migration logic to useMultiTrack hook:
- When loading tracks from localStorage, check if height < DEFAULT_TRACK_HEIGHT
- Automatically upgrade old heights (120px, 150px) to new default (180px)
- Preserves custom heights that are already >= 180px

This fixes the inline style issue where existing tracks had
style="height: 120px" that was cutting off the level meter.

After this update, refreshing the page will automatically upgrade
all existing tracks to the new height without losing any data.

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

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

lib/hooks/useMultiTrack.ts

@@ -2,6 +2,7 @@ import { useState, useCallback, useEffect } from 'react';
 import type { Track } from '@/types/track';
 import { createTrack, createTrackFromBuffer } from '@/lib/audio/track-utils';
 import { createEffectChain } from '@/lib/audio/effects/chain';
+import { DEFAULT_TRACK_HEIGHT } from '@/types/track';
 
 const STORAGE_KEY = 'audio-ui-multi-track';
 
@@ -29,6 +30,7 @@ export function useMultiTrack() {
         return parsed.map((t: any) => ({
           ...t,
           name: String(t.name || 'Untitled Track'), // Ensure name is always a string
+          height: t.height && t.height >= DEFAULT_TRACK_HEIGHT ? t.height : DEFAULT_TRACK_HEIGHT, // Migrate old heights
           audioBuffer: null, // Will need to be reloaded
           effectChain: t.effectChain || createEffectChain(`${t.name} Effects`), // Restore effect chain or create new
           selection: t.selection || null, // Initialize selection