fix(frontend): resolve build-blocking lint and type errors

next build runs ESLint + full type-check and fails on errors (not just
warnings) - the production build was broken. Fixes: unused imports/vars,
any-typed diff/preference lookups replaced with the actual union/Record
types, ternary-as-statement flagged by no-unused-expressions, unescaped
JSX entities, a UserPreferences interface missing two fields the video
editor already reads/writes at runtime, and an import of
ColumnVisibilityState which @tanstack/react-table does not export
(the real name is VisibilityState).

frontend/src/app/(dashboard)/linting/page.tsx

@@ -45,8 +45,6 @@ const RULE_CATALOG = [
   { code: 'REMOTE_CONFLICT',            severity: 'ERROR',   label: 'Remote conflict',         desc: 'The video was modified on YouTube after the last sync — local and remote diverged.' },
 ] as const;
 
-type RuleCode = typeof RULE_CATALOG[number]['code'];
-
 // ── Helpers ───────────────────────────────────────────────────────────────────
 
 function SevBadge({ sev }: { sev: string }) {
@@ -156,7 +154,7 @@ export default function LintingPage() {
   function toggleSelect(id: string) {
     setSelected((prev) => {
       const next = new Set(prev);
-      next.has(id) ? next.delete(id) : next.add(id);
+      if (next.has(id)) next.delete(id); else next.add(id);
       return next;
     });
   }
@@ -172,7 +170,7 @@ export default function LintingPage() {
   function toggleFix(id: string) {
     setExpandedFix((prev) => {
       const next = new Set(prev);
-      next.has(id) ? next.delete(id) : next.add(id);
+      if (next.has(id)) next.delete(id); else next.add(id);
       return next;
     });
   }