feat: disable localStorage persistence and add auto-load last project

**useMultiTrack.ts:**
- Removed localStorage persistence (tracks, effects, automation)
- Now relies entirely on IndexedDB via project management system
- Simpler state management without dual persistence

**AudioEditor.tsx:**
- Store last project ID in localStorage when saving
- Auto-load last project on page mount
- Only runs once per session with hasAutoLoaded flag
- Falls back to empty state if project can't be loaded

**Benefits:**
- No more conflicts between localStorage and IndexedDB
- Effects and automation properly persisted
- Seamless experience - reload page and your project is ready
- Single source of truth (IndexedDB)

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

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

components/editor/AudioEditor.tsx

@@ -911,6 +911,9 @@ export function AudioEditor() {
 
       setCurrentProjectId(projectId);
 
+      // Save last project ID to localStorage for auto-load on next visit
+      localStorage.setItem('audio-ui-last-project', projectId);
+
       addToast({
         title: 'Project Saved',
         description: `"${currentProjectName}" saved successfully`,
@@ -1009,6 +1012,29 @@ export function AudioEditor() {
     }
   }, [loadTracks, addToast]);
 
+  // Auto-load last project on mount
+  const [hasAutoLoaded, setHasAutoLoaded] = React.useState(false);
+  React.useEffect(() => {
+    if (hasAutoLoaded) return; // Only run once
+
+    const loadLastProject = async () => {
+      const lastProjectId = localStorage.getItem('audio-ui-last-project');
+      if (lastProjectId) {
+        try {
+          console.log('[Auto-load] Loading last project:', lastProjectId);
+          await handleLoadProject(lastProjectId);
+        } catch (error) {
+          console.error('[Auto-load] Failed to load last project:', error);
+          // Clear invalid project ID
+          localStorage.removeItem('audio-ui-last-project');
+        }
+      }
+      setHasAutoLoaded(true);
+    };
+
+    loadLastProject();
+  }, [hasAutoLoaded, handleLoadProject]);
+
   // Delete project
   const handleDeleteProject = React.useCallback(async (projectId: string) => {
     try {