fix: replace flyout profile card with logout slider, i18n auth errors

- Replace static account card in mobile flyout with swipe-to-logout widget
- Remove redundant logout button from flyout bottom
- Make LogoutButton full-width via class prop and dynamic maxSlide
- Extract clean GraphQL error messages instead of raw JSON in all auth forms
- Add i18n keys for known backend errors (invalid credentials, email taken, invalid token)

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

packages/frontend/src/lib/i18n/locales/en.ts

@@ -168,6 +168,7 @@ export default {
       no_account: "Don't have an account?",
       sign_up_link: "Sign up now",
       error: "Heads Up!",
+      error_invalid_credentials: "Invalid email or password.",
     },
     signup: {
       title: "Create Account",
@@ -194,6 +195,7 @@ export default {
       have_account: "Already have an account?",
       sign_in_link: "Sign in here",
       error: "Heads Up!",
+      error_email_taken: "This email address is already registered.",
       agree_error: "You must confirm our terms of service and your age.",
       password_error: "The password has to match the confirmation password.",
       toast_register: "A verification email has been sent to {email}!",
@@ -221,6 +223,7 @@ export default {
       resetting: "Resetting...",
       reset: "Reset",
       error: "Heads Up!",
+      error_invalid_token: "This reset link is invalid or has expired.",
       password_error: "The password has to match the confirmation password.",
       toast_reset: "Your password has been reset!",
     },