Fix transcribe form blocked by required textarea validation

The hidden textarea still had required=true, so browser-native form
validation silently blocked submit when no audio was the only input.

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

assets/js/tools.js

@@ -164,6 +164,7 @@ function setTool(toolName) {
   els.transcribeLangControl.classList.toggle('is-hidden', toolName !== 'transcribe');
   els.input.classList.toggle('is-hidden', toolName === 'transcribe');
   els.inputLabel.classList.toggle('is-hidden', toolName === 'transcribe');
+  els.input.required = toolName !== 'transcribe';
   resetUpload();
   resetAliases();
   resetAudio();