Add ATC thumbnail URL resolution for R2 storage

- Add resolveImgUrl() to prepend https://atc.tzzrdeck.me/ to relative paths
- Update getImg() and getFullImg() to use the new resolver
- Enables thumbnail display for attachments stored in R2 bucket

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

deck-frontend/src/utils/i18n.ts

@@ -22,10 +22,21 @@ export function resolveGroupName(mrf: string | undefined, nameMap: Map<string, s
   return nameMap.get(mrf) || mrf.slice(0, 8);
 }
 
+const ATC_BASE = 'https://atc.tzzrdeck.me';
+
+function resolveImgUrl(url: string | undefined): string {
+  if (!url) return '';
+  // Relative paths (e.g., "thumbs/xxx.png") need ATC base
+  if (url && !url.startsWith('http')) {
+    return `${ATC_BASE}/${url}`;
+  }
+  return url;
+}
+
 export function getImg(tag: Tag): string {
-  return tag.img_thumb_url || '';
+  return resolveImgUrl(tag.img_thumb_url);
 }
 
 export function getFullImg(tag: Tag): string {
-  return tag.img_url || tag.img_thumb_url || '';
+  return resolveImgUrl(tag.img_url) || resolveImgUrl(tag.img_thumb_url);
 }