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/routes/signup/+page.svelte

@@ -48,7 +48,8 @@
       toast.success($_("auth.signup.toast_register", { values: { email } }));
       goto("/login");
     } catch (err: any) {
-      error = err.message;
+      const raw = err.response?.errors?.[0]?.message ?? err.message;
+      error = raw === "Email already registered" ? $_("auth.signup.error_email_taken") : raw;
       isError = true;
     } finally {
       isLoading = false;