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/discord-gateway/src/modules/command-handler/media.handler.ts

@@ -1,8 +1,8 @@
 import { randomUUID } from "node:crypto";
+import { StreamType } from "@discordjs/voice";
 import type { Client } from "discord.js-selfbot-v13";
 import type { CommandMessage, CommandReply } from "../../shared/index.js";
 import { createChildLogger } from "../../shared/logger/index.js";
-import { StreamType } from "@discordjs/voice";
 import {
   extractMediaInfo,
   resolveMediaUrl,