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/components/shared/SavedViewManager.tsx

@@ -5,7 +5,7 @@ import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { X, Plus, Pin, PinOff, Trash2, ChevronUp, ChevronDown, Loader2 } from 'lucide-react';
 import {
   fetchSavedViews, createSavedView, updateSavedView, deleteSavedView,
-  type SavedView, type VideosQuery, type UserPreferences,
+  type SavedView, type VideosQuery,
 } from '@/lib/api';
 import styles from './SavedViewManager.module.css';