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/ai-moderation/moderationBuilders.ts

@@ -5,9 +5,9 @@
  * Used by both mediaAnalysisClient.ts and moderationOrchestrator.ts.
  */
 
+import { renderDiscordMentions } from "../message-capture/messageMetadata.js";
 import { messageStore } from "../message-capture/messageStore.js";
 import type { MessageRecord } from "../message-capture/types.js";
-import { renderDiscordMentions } from "../message-capture/messageMetadata.js";
 import { sanitizeDiscordTokens } from "./discordTokens.js";
 
 /** Simple XML-escaping for content text. */