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)/settings/page.tsx

@@ -483,7 +483,7 @@ export default function SettingsPage() {
       <section className={styles.section}>
         <h2 className={styles.sectionTitle}>Publishing Schedule</h2>
         <p className={styles.sectionDesc}>
-          Define the time slots when videos are allowed to publish. Used by the "Next free slot" feature in the video editor.
+          Define the time slots when videos are allowed to publish. Used by the &quot;Next free slot&quot; feature in the video editor.
         </p>
 
         <div className={styles.scheduleBox}>
@@ -581,7 +581,7 @@ export default function SettingsPage() {
                 />
                 <span>Show Canva link in video editor</span>
                 <span className={styles.toggleDesc}>
-                  Adds a link next to YouTube Studio that searches Canva for the video's Game Title.
+                  Adds a link next to YouTube Studio that searches Canva for the video&apos;s Game Title.
                 </span>
               </label>
             ) : (
@@ -601,7 +601,7 @@ export default function SettingsPage() {
                 />
                 <span>Show deleted videos tab</span>
                 <span className={styles.toggleDesc}>
-                  Reveals a "Deleted" tab in the video list for videos removed from YouTube. They are kept locally for 30 days before being permanently deleted.
+                  Reveals a &quot;Deleted&quot; tab in the video list for videos removed from YouTube. They are kept locally for 30 days before being permanently deleted.
                 </span>
               </label>
             ) : (