fix(mobile): useModalStyle on 3 more dialogs, GPU hint animated bg, document untested fixes

- DeviceVerification, InviteUserPrompt, LogoutDialog: apply useModalStyle for
  fullscreen on mobile, capped box on desktop
- chatBackground: inject willChange + contain:paint on animated variants to
  promote compositor layer and prevent descendant repaint flickering (Bug #2)
- LOTUS_BUGS.md: mark #7-#10 FIXED/UNTESTED, #2 FIXED/UNTESTED, chatBackground
  backgroundColor values as PATTERN CONTENT EXCEPTION

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

src/app/features/lotus/chatBackground.ts

@@ -493,5 +493,10 @@ export const getChatBg = (
     const { animation: _anim, ...rest } = style;
     return rest;
   }
+  // For animated backgrounds, promote the element to its own compositor layer so
+  // background-position keyframes don't trigger repaints on descendant elements.
+  if (style.animation) {
+    return { ...style, willChange: 'background-position', contain: 'paint' };
+  }
   return style;
 };