add "text only" as another advanced option for captions format

ui/src/app/app.html

@@ -232,6 +232,9 @@
                       }
                     </select>
                   </div>
+                  @if (subtitleFormat === 'txt') {
+                    <div class="form-text">TXT is generated from SRT by stripping timestamps and cue numbers.</div>
+                  }
                 </div>
                 <div class="col-md-4">
                   <div class="input-group">

ui/src/app/app.ts

@@ -102,6 +102,7 @@ export class App implements AfterViewInit, OnInit {
   faTachometerAlt = faTachometerAlt;
   subtitleFormats = [
     { id: 'srt', text: 'SRT' },
+    { id: 'txt', text: 'TXT (Text only)' },
     { id: 'vtt', text: 'VTT' },
     { id: 'ttml', text: 'TTML' }
   ];