chore(lint): biome cleanup across services — format, sort imports, drop unused

- discord-gateway: 74 lint errors -> 0 (format, import sorting, unused
  imports/vars, dead breath var)
- backend: format + sort imports (11 warnings left: noExplicitAny)
- frontend: remove unused imports, drop dead breathing var, fix
  useExhaustiveDependencies (scroll keyed on messages), a11y biome-ignore
  for drag surface + stopPropagation container (mouse-only gestures)
- remaining warnings are false positives: index keys on static lists,
  <img> in static export (next/image unsupported), noExplicitAny

tsc --noEmit clean on all 3 services; vitest green (60+36).

services/frontend/src/hooks/use-messages.ts

@@ -138,7 +138,12 @@ export function useMessageDetail(id: string | null) {
       if (!cid) return [];
       // messageId filter: attachment list must show only this message's
       // images, not the latest images from everyone in the channel.
-      const res = await messagesApi.getAttachments(cid, 10, undefined, id ?? "");
+      const res = await messagesApi.getAttachments(
+        cid,
+        10,
+        undefined,
+        id ?? "",
+      );
       return res.data;
     },
   );