feat(account): add editable phone and password change with validation

- Add editable phone field in /account General tab with UK phone
  validation
- Create PUT /api/user/change-password endpoint in backend
- Add zxcvbn password strength meter to change password modal
- Add "passwords don't match" validation message to both /account and
  /register
- Fix navbar logout reactivity with invalidateAll and $derived values
- Fix a11y warnings: add labels, roles, and keyboard handlers
- Remove unused CSS from account page

frontend/src/lib/stores/auth.svelte.ts

@@ -146,7 +146,7 @@ class AuthStore {
 	// inside AuthStore
 	logout = () => {
 		this.clearAuth();
-		goto('/');
+		goto('/', { invalidateAll: true });
 	};
 
 	private clearAuth() {