Fix watermark transparency: use gr.File instead of gr.Image

gr.Image may convert/process uploads losing alpha channel. gr.File preserves original PNG with transparency.

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

app.py

@@ -777,7 +777,7 @@ with gr.Blocks(title="Element-8 Video", delete_cache=(3600, 7200)) as demo:  # c
                     remove_music = gr.Checkbox(label="Remove Music", value=False)
                 with gr.Row():
                     add_subtitles = gr.Checkbox(label="Add Subtitles (Whisper)", value=False)
-                    watermark = gr.Image(label="Watermark PNG (full-video size, position in your editor)", type="filepath", sources=["upload"])
+                    watermark = gr.File(label="Watermark PNG (full-video size, position in your editor)", file_types=[".png"])
                 negative_prompt = gr.Textbox(
                     label="Negative Prompt",
                     value=DEFAULT_NEGATIVE_PROMPT,