feat: implement Phase 12.3 - Project Export/Import

Added full export/import functionality for projects:

Export Features:
- Export button for each project in Projects dialog
- Downloads project as JSON file with all data
- Includes tracks, audio buffers, effects, automation, settings
- Filename format: {project_name}_{timestamp}.json

Import Features:
- Import button in Projects dialog header
- File picker for .json files
- Automatically generates new project ID to avoid conflicts
- Appends "(Imported)" to project name
- Preserves all project data

This enables:
- Backup of projects outside the browser
- Sharing projects with collaborators
- Migration between computers/browsers
- Version snapshots at different stages

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

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

components/dialogs/ProjectsDialog.tsx

@@ -1,7 +1,7 @@
 'use client';
 
 import * as React from 'react';
-import { X, Plus, Trash2, Copy, FolderOpen } from 'lucide-react';
+import { X, Plus, Trash2, Copy, FolderOpen, Download, Upload } from 'lucide-react';
 import { Button } from '@/components/ui/Button';
 import type { ProjectMetadata } from '@/lib/storage/db';
 import { formatDuration } from '@/lib/audio/decoder';
@@ -14,6 +14,8 @@ export interface ProjectsDialogProps {
   onLoadProject: (projectId: string) => void;
   onDeleteProject: (projectId: string) => void;
   onDuplicateProject: (projectId: string) => void;
+  onExportProject: (projectId: string) => void;
+  onImportProject: () => void;
 }
 
 export function ProjectsDialog({
@@ -24,6 +26,8 @@ export function ProjectsDialog({
   onLoadProject,
   onDeleteProject,
   onDuplicateProject,
+  onExportProject,
+  onImportProject,
 }: ProjectsDialogProps) {
   if (!open) return null;
 
@@ -44,6 +48,15 @@ export function ProjectsDialog({
         <div className="flex items-center justify-between p-6 border-b border-border">
           <h2 className="text-lg font-semibold text-foreground">Projects</h2>
           <div className="flex items-center gap-2">
+            <Button
+              onClick={onImportProject}
+              variant="outline"
+              size="sm"
+              className="gap-2"
+            >
+              <Upload className="h-4 w-4" />
+              Import
+            </Button>
             <Button
               onClick={onNewProject}
               variant="default"
@@ -111,6 +124,13 @@ export function ProjectsDialog({
                       >
                         Open
                       </button>
+                      <button
+                        onClick={() => onExportProject(project.id)}
+                        className="p-1.5 text-muted-foreground hover:text-foreground hover:bg-accent rounded transition-colors"
+                        title="Export project"
+                      >
+                        <Download className="h-4 w-4" />
+                      </button>
                       <button
                         onClick={() => onDuplicateProject(project.id)}
                         className="p-1.5 text-muted-foreground hover:text-foreground hover:bg-accent rounded transition-colors"