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/lib/audio/pcm-player.ts

@@ -66,7 +66,12 @@ export class PcmPlayer {
     if (!this.started || samples.length === 0) return;
     let ring = this.rings.get(userIdHash);
     if (!ring) {
-      ring = { data: new Float32Array(RING_LEN), write: 0, readPos: 0, lastActive: Date.now() };
+      ring = {
+        data: new Float32Array(RING_LEN),
+        write: 0,
+        readPos: 0,
+        lastActive: Date.now(),
+      };
       this.rings.set(userIdHash, ring);
     }
     ring.lastActive = Date.now();