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/components/messages/attachments-grid.tsx

@@ -12,7 +12,10 @@ export function AttachmentsGrid({ attachments }: AttachmentsGridProps) {
   return (
     <div className="grid grid-cols-2 gap-2">
       {attachments.map((att) => (
-        <div key={att.id} className="glass rounded-lg overflow-hidden group relative">
+        <div
+          key={att.id}
+          className="glass rounded-lg overflow-hidden group relative"
+        >
           {att.type?.startsWith("image/") ? (
             <img
               src={att.uploaded_url || att.discord_url}