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>

lib/storage/projects.ts

@@ -210,3 +210,64 @@ export async function duplicateProject(sourceProjectId: string, newName: string)
   await saveProject(newProject);
   return newId;
 }
+
+/**
+ * Export project as JSON file
+ */
+export async function exportProjectAsJSON(projectId: string): Promise<void> {
+  const project = await loadProject(projectId);
+  if (!project) throw new Error('Project not found');
+
+  // Convert the project to JSON
+  const json = JSON.stringify(project, null, 2);
+
+  // Create blob and download
+  const blob = new Blob([json], { type: 'application/json' });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = `${project.metadata.name.replace(/[^a-z0-9]/gi, '_').toLowerCase()}_${Date.now()}.json`;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}
+
+/**
+ * Import project from JSON file
+ */
+export async function importProjectFromJSON(file: File): Promise<string> {
+  return new Promise((resolve, reject) => {
+    const reader = new FileReader();
+
+    reader.onload = async (e) => {
+      try {
+        const json = e.target?.result as string;
+        const project = JSON.parse(json) as ProjectData;
+
+        // Generate new ID to avoid conflicts
+        const newId = generateProjectId();
+        const now = Date.now();
+
+        const importedProject: ProjectData = {
+          ...project,
+          metadata: {
+            ...project.metadata,
+            id: newId,
+            name: `${project.metadata.name} (Imported)`,
+            createdAt: now,
+            updatedAt: now,
+          },
+        };
+
+        await saveProject(importedProject);
+        resolve(newId);
+      } catch (error) {
+        reject(new Error('Failed to parse project file'));
+      }
+    };
+
+    reader.onerror = () => reject(new Error('Failed to read file'));
+    reader.readAsText(file);
+  });
+}