build: 恢复前端类型检查

vue-tsc 不支持 TypeScript 7，前端单独用 TS 5.9 + vue-tsc（服务端仍是 TS 7），
根目录 bun run typecheck 同时检查两边。修掉前端原有的 8 个类型错误（未使用的变量、类型标注）。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/pages/Submissions.vue

@@ -115,10 +115,8 @@ import FlagCell from "../components/submissions/FlagCell.vue"
 import ExpandedSubTable from "../components/submissions/ExpandedSubTable.vue"
 
 import { submission } from "../store/submission"
-import { taskTab, challengeDisplay } from "../store/task"
-import { step } from "../store/tutorial"
 import { html as eHtml, css as eCss, js as eJs } from "../store/editors"
-import { roleAdmin, roleSuper, user } from "../store/user"
+import { roleAdmin, roleSuper } from "../store/user"
 
 const route = useRoute()
 const router = useRouter()