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/me/+page.svelte

@@ -87,7 +87,7 @@
       toast.success($_("me.settings.toast_update"));
       invalidateAll();
     } catch (err: any) {
-      profileError = err.message;
+      profileError = err.response?.errors?.[0]?.message ?? err.message;
       isProfileError = true;
     } finally {
       isProfileLoading = false;
@@ -111,7 +111,7 @@
       invalidateAll();
       password = confirmPassword = "";
     } catch (err: any) {
-      securityError = err.message;
+      securityError = err.response?.errors?.[0]?.message ?? err.message;
       isSecurityError = true;
     } finally {
       isSecurityLoading = false;