Hide UI elements until dark mode active to prevent FOUC

Elements are hidden with opacity:0 initially, then shown
when .dark class is applied. Shows white flash only.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app_diffusers.py

@@ -580,6 +580,14 @@ with gr.Blocks(
         input_shadow_focus='0 0 0 4px rgba(0, 113, 227, 0.15)',
     ),
     css=apple_css,
+    head="""<style>
+.gradio-container .block, .gradio-container textarea, .gradio-container button, .gradio-container select, .gradio-container input, .gradio-container .input-section, .gradio-container .output-section {
+    opacity: 0 !important;
+}
+.dark .gradio-container .block, .dark .gradio-container textarea, .dark .gradio-container button, .dark .gradio-container select, .dark .gradio-container input, .dark .gradio-container .input-section, .dark .gradio-container .output-section {
+    opacity: 1 !important;
+}
+</style>""",
 ) as demo:
     # Two-column layout - variant='panel' prevents responsive stacking
     with gr.Row(equal_height=False, variant="panel", elem_id="main-row"):