Fix stepper checkmarks, error recovery, radio button spacing

- Both steppers now show green checkmark on final step when completed
  instead of leaving it highlighted as 'current' (blue)
- Stems error is cleared at the start of every generate_stems attempt,
  not just on success — no stale error messages after retry
- Radio buttons in stems section use same compact spacing as checkboxes

app.py

@@ -1214,6 +1214,7 @@ def admin_request(rid):
 
         elif action == 'generate_stems':
             # Queue a stem extraction job for the selected MP3 (Version A or B).
+            # Clear any previous error at the start of every attempt.
             api_key = get_musicgpt_api_key()
             if not api_key:
                 flash('MusicGPT API key is not configured.', 'error')
@@ -1221,6 +1222,8 @@ def admin_request(rid):
             if req.get('stems_status') in ('IN_QUEUE', 'IN_PROGRESS'):
                 flash('A stems extraction is already in progress.', 'error')
                 return redirect(url_for('admin_request', rid=rid))
+            # Clear prior error so the operator doesn't see a stale message after retry.
+            update_request(rid, stems_error=None)
             # Operator selects which version (A or B) to extract stems from.
             selected = request.form.get('stems_source', '').strip()
             if selected == 'a':