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/message-capture/retentionDb.ts

@@ -1,6 +1,6 @@
-import { createChildLogger, type Logger } from "@/shared/logger/index";
 import { and, eq, isNull, or } from "drizzle-orm";
 import type { NodePgDatabase } from "drizzle-orm/node-postgres";
+import { createChildLogger, type Logger } from "@/shared/logger/index";
 import type * as schema from "../../shared/database/schema.js";
 import { retentionPoliciesTable } from "../../shared/database/schema.js";
 import type { RetentionPolicy } from "../message-capture/types.js";
@@ -45,8 +45,7 @@ export class RetentionDb {
             ),
           );
 
-        if (channelRows.length > 0)
-          return channelRows[0] as RetentionPolicy;
+        if (channelRows.length > 0) return channelRows[0] as RetentionPolicy;
       }
 
       // Fall back to the guild default (channel_id IS NULL)