fix(ui): use gr.JSON for metadata output instead of gr.Code

gr.Code(language='json') called .strip() on the postprocess value,
expecting a pre-stringified JSON blob. backend.dispatch returns
meta as a Python dict, so passing it directly crashed with
"'dict' object has no attribute 'strip'". gr.JSON accepts the dict
natively and renders it as a syntax-highlighted, expandable tree —
which is closer to what the wireframes show in the Metadata panel
anyway.

Caught while running the live app post-pipeline-fix: the audio file
saved correctly to ./output/<uuid>.wav, but Gradio's output postprocess
crashed on the metadata component.

ui.py

@@ -65,9 +65,9 @@ def build_generate_tab() -> dict[str, gr.components.Component]:
                 type="filepath",
                 interactive=False,
             )
-            components["output_meta"] = gr.Code(
-                label="Metadata",
-                language="json",
-            )
+            # gr.JSON renders a dict directly as a syntax-highlighted, expandable
+            # tree. gr.Code(language="json") refuses dicts — it requires a
+            # pre-stringified blob — and crashes with "'dict' has no .strip()".
+            components["output_meta"] = gr.JSON(label="Metadata")
 
     return components