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/components/logout-button/logout-button.svelte

@@ -11,15 +11,17 @@
   interface Props {
     user: User;
     onLogout: () => void;
+    class?: string;
   }
 
-  let { user, onLogout }: Props = $props();
+  let { user, onLogout, class: className = "" }: Props = $props();
 
+  let container: HTMLDivElement;
   let isDragging = $state(false);
   let slidePosition = $state(0);
   let startX = 0;
   let currentX = 0;
-  let maxSlide = 117; // Maximum slide distance
+  let maxSlide = $derived(container ? container.offsetWidth - 40 : 117);
   let threshold = 0.75; // 70% threshold to trigger logout
 
   // Calculate slide progress (0 to 1)
@@ -102,9 +104,10 @@
 </script>
 
 <div
+  bind:this={container}
   class="relative h-10 w-40 rounded-full bg-muted/30 overflow-hidden select-none transition-all duration-300 bg-muted/40 shadow-lg shadow-accent/10 {isDragging
     ? 'cursor-grabbing'
-    : ''}"
+    : ''} {className}"
   style="background: linear-gradient(90deg, 
 		oklch(var(--primary) / 0.3) 0%, 
 		oklch(var(--primary) / 0.3) {(1 - slideProgress) * 100}%,