Consistency sweep: fix switcher layout, gaps, buttons, empty states

- Home: mode switcher moved to its own row (no longer crammed next to
  title on mobile), hide-watched simplified to text-only toggle
- Home/History/Discovery: pagination buttons text-sm → text-xs, page
  counter text-sm → text-xs
- Liked/Downloads/SearchResults: top-level gap-8 → gap-6
- Liked: refresh button px-4 py-2 text-sm → px-3 py-1.5 text-xs
- Empty states: standardize to text-zinc-500 text-sm across Queue,
  ContinueWatching, History, Following, Discovery, Liked
- Following: "Latest uploads" tab label → "Feed"
- Home: remove -mt-3 hacks from mode description rows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/ContinueWatching.jsx

@@ -25,7 +25,7 @@ export default function ContinueWatchingPage() {
             </svg>
           </div>
           <div>
-            <p className="text-zinc-300 font-medium">Nothing in progress</p>
+            <p className="text-zinc-500 text-sm">Nothing in progress</p>
             <p className="text-zinc-500 text-sm mt-1">
               Videos you've started but not finished will appear here.
             </p>