Activate subtitle track when lang selected from dropdown

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

frontend/src/pages/Watch.jsx

@@ -1014,7 +1014,17 @@ export default function Watch() {
                   <>
                     <select
                       value={selectedSubLang}
-                      onChange={(e) => setSelectedSubLang(e.target.value)}
+                      onChange={(e) => {
+                        const lang = e.target.value;
+                        setSelectedSubLang(lang);
+                        const v = videoRef.current;
+                        if (v) {
+                          for (let i = 0; i < v.textTracks.length; i++) {
+                            v.textTracks[i].mode =
+                              lang && v.textTracks[i].language === lang ? "showing" : "disabled";
+                          }
+                        }
+                      }}
                       className="bg-zinc-800 text-zinc-300 text-xs rounded-full px-3 py-2 border border-zinc-700 focus:outline-none focus:border-accent"
                     >
                       <option value="">No subtitles</option>