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/dashboard/dashboard.repository.ts

@@ -1,3 +1,6 @@
+import type { SQL } from "drizzle-orm";
+import { sql } from "drizzle-orm";
+import { getDatabase } from "../../shared/database/index.js";
 import {
   pgChannelCulturesTable,
   pgMessagesTable,
@@ -5,9 +8,6 @@ import {
   pgUserReputationsTable,
   pgVoiceRecordingsTable,
 } from "../../shared/index.js";
-import type { SQL } from "drizzle-orm";
-import { sql } from "drizzle-orm";
-import { getDatabase } from "../../shared/database/index.js";
 import type { ListUsersQuery } from "./dashboard.service.js";
 
 export class DashboardRepository {