feat(auth): add SignupForm component with validation and password toggle

Form with name/email/password fields, client-side validation,
inline error messages, and password show/hide toggle.
Uses native form action for progressive enhancement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -25,6 +25,7 @@
 		"@tailwindcss/vite": "^4.2.2",
 		"@testing-library/jest-dom": "^6.9.1",
 		"@testing-library/svelte": "^5.3.1",
+		"@testing-library/user-event": "^14.6.1",
 		"@types/node": "^25.5.0",
 		"@vitest/ui": "^4.1.2",
 		"jsdom": "^29.0.1",