Add hackathon track/sponsor/achievement tags, remove blank line in tags block

Update README.md

Update README: hackathon tags, demo video, social post, sdk:gradio

switch to sdk: docker + app_custom.py (React UI); avoids Gradio SDK port conflict

switch app_file to app.py (Gradio Blocks); move css/theme/js to Blocks constructor

Deploy: French Coach app (MiniCPM4.1-8B ZeroGPU + React frontend)

initial commit