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/backend/src/modules/media/media.service.ts

@@ -1,3 +1,7 @@
+import {
+  createChildLogger,
+  tryCommandThenFallback,
+} from "../../shared/commandHelper.js";
 import {
   COMMAND_MEDIA_QUEUE,
   COMMAND_MEDIA_SKIP,
@@ -5,10 +9,6 @@ import {
   COMMAND_MEDIA_VOLUME,
   MEDIA_STATUS_KEY,
 } from "../../shared/index.js";
-import {
-  createChildLogger,
-  tryCommandThenFallback,
-} from "../../shared/commandHelper.js";
 import { publishCommand, readRedisStatus } from "../../shared/redis/index.js";
 
 const logger = createChildLogger("media.service");