feat: [US-013] - Create new project

Add NewProjectButton component with modal dialog for creating new projects:
- Button displays on dashboard with plus icon
- Modal with project name input
- Creates project in Supabase with empty flowchart_data
- Redirects to /editor/[projectId] on success
- Error handling with user feedback

Also fixes lint error in signup page (setState in effect).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/signup/page.tsx

@@ -8,19 +8,14 @@ import { createClient } from '@/lib/supabase/client'
 export default function SignupPage() {
   const router = useRouter()
   const searchParams = useSearchParams()
-  const [email, setEmail] = useState('')
+  // Pre-fill email if provided in URL (from invite link)
+  const [email, setEmail] = useState(searchParams.get('email') ?? '')
   const [password, setPassword] = useState('')
   const [confirmPassword, setConfirmPassword] = useState('')
   const [error, setError] = useState<string | null>(null)
   const [loading, setLoading] = useState(false)
 
   useEffect(() => {
-    // Pre-fill email if provided in URL (from invite link)
-    const emailParam = searchParams.get('email')
-    if (emailParam) {
-      setEmail(emailParam)
-    }
-
     // Handle invite/signup token from URL hash
     // Supabase adds tokens to the URL hash after redirect
     const handleTokenFromUrl = async () => {