feat: improve mobile responsiveness across UI components

- Add responsive layout for action buttons (stack vertically on mobile, horizontal on sm+)
- Adjust card max-widths for better mobile display (LocationCard, MultiBillEdit)
- Improve button text sizing with responsive classes (text-sm on mobile, text-xs on sm+)
- Remove width constraints on file inputs and info boxes for full-width mobile layout
- Fine-tune icon positioning for better visual alignment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/ui/InfoBox.tsx

@@ -11,7 +11,7 @@ export const InfoBox: FC<{
     const t = useTranslations("info-box");
 
   return (
-    <details className="group border border-gray-800 rounded-lg p-2 mb-1 w-full max-w-md">
+    <details className="group border border-gray-800 rounded-lg p-2 mb-1 w-full">
       <summary className="flex cursor-pointer items-center justify-between">
         <span className="font-bold"><QuestionMarkCircleIcon className="w-5 h-5 inline mr-1 mt-[-.3em]" /> {title ?? t("default-title")}</span>
         <span className="ml-2 text-sm text-gray-500 group-open:hidden"><ChevronDownIcon className="w-5 h-5 inline" /></span>