Migrate from Firebase to file-based storage system

- Remove Firebase dependency and all Firebase code
- Implement local file-based storage with encryption
- Create custom authentication system with password hashing
- Add API routes for auth (register/login) and data operations
- Create FileStorageService for server-side file I/O
- Add AuthService for client-side authentication wrapper
- Update all components to use new storage system
- Add comprehensive documentation (FILE_STORAGE.md, MIGRATION_SUMMARY.md)
- Improve security with AES-GCM encryption and SHA-256 password hashing
- Add ErrorBoundary component for better error handling
- Fix multiple bugs: date serialization, transaction deletion, envelope calculations

src/app/api/auth/register/route.ts

@@ -0,0 +1,27 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { FileStorageService } from '@/lib/fileStorage';
+
+export async function POST(request: NextRequest) {
+  try {
+    const { email, password } = await request.json();
+
+    if (!email || !password) {
+      return NextResponse.json(
+        { error: 'Email and password are required' },
+        { status: 400 }
+      );
+    }
+
+    const user = await FileStorageService.registerUser(email, password);
+
+    return NextResponse.json({ 
+      success: true,
+      user 
+    });
+  } catch (error: any) {
+    return NextResponse.json(
+      { error: error.message || 'Registration failed' },
+      { status: 400 }
+    );
+  }
+}