Develop (#55)

feat: optimize dashboard/search UX and simplify extension architecture

Redesign Dashboard with compact tool grid and recently used tools
Improve TopBar search UX with Cmd/Ctrl+K shortcut and better history navigation
Reorganize project structure into src/
Migrate i18n from react-i18next to chrome.i18n
Remove runtime language switch and settings page
Remove HTML/Markdown conversion tools
Clean up unused code, dead animations, redundant comments, and imports
Improve component consistency with shadcn/ui patterns
Replace hardcoded strings/colors with i18n tokens and theme tokens
Add comprehensive project documentation and coding standards
Fix CI artifact upload workflow and multiple TypeScript/test issues

Includes various refactors, UI polish, i18n cleanup, CI improvements,
and maintenance updates across the codebase.

src/pages/StorageCleaner/ErrorDisplay.tsx

@@ -0,0 +1,35 @@
+import { AlertCircle } from 'lucide-react';
+import { useI18n } from '@/utils/chromeI18n';
+import { cn } from '@/lib/utils';
+
+interface ErrorDisplayProps extends React.HTMLAttributes<HTMLDivElement> {
+  error: string;
+}
+
+export default function ErrorDisplay({ error, className, ...props }: ErrorDisplayProps) {
+  const { t } = useI18n('storageCleaner');
+
+  return (
+    <div
+      className={cn(
+        'flex flex-col items-center justify-center py-8 min-h-[240px] sm:min-h-[360px] p-4 text-center',
+        className,
+      )}
+      {...props}
+    >
+      <div className="w-full max-w-xs flex flex-col items-center justify-center rounded-xl p-5 border border-destructive/20 bg-destructive/5 shadow-sm">
+        <div className="flex h-10 w-10 items-center justify-center rounded-full bg-destructive/10 text-destructive mb-3.5 shrink-0">
+          <AlertCircle className="h-5 w-5" />
+        </div>
+
+        <p className="text-sm font-semibold leading-relaxed text-destructive break-all px-1 mb-2">
+          {error}
+        </p>
+
+        <p className="text-xs font-medium leading-relaxed text-muted-foreground/90 px-2">
+          {t('storageCleaner:errorStandardOnly')}
+        </p>
+      </div>
+    </div>
+  );
+}