Fix async generator syntax errors in tab files

Changed return statements to yield + return in async generator
functions. Python doesn't allow 'return' with a value in generators.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/ui/tabs/musicgen_tab.py

@@ -189,7 +189,7 @@ def create_musicgen_tab(
             prompt, variant, duration, temperature, cfg_coef, top_k, top_p, seed, melody
         ):
             if not prompt:
-                return (
+                yield (
                     gr.update(value="Please enter a prompt"),
                     gr.update(),
                     gr.update(),
@@ -197,6 +197,7 @@ def create_musicgen_tab(
                     gr.update(),
                     gr.update(),
                 )
+                return
 
             # Update status
             yield (