Fix flux_train_ui startup on Gradio 6 by removing removed Image kwargs

[iqdoctor]

Fixes #776.

Summary

• remove show_share_button=False from the gr.Image(...) used in flux_train_ui.py
• remove show_download_button=False from the same component

Why

Gradio 6.x no longer accepts these kwargs, so flux_train_ui crashes while building the UI with:

TypeError: Image.__init__() got an unexpected keyword argument 'show_share_button'

Validation

env -u ALL_PROXY -u all_proxy -u HTTP_PROXY -u HTTPS_PROXY -u WS_PROXY -u WSS_PROXY -u http_proxy -u https_proxy \
  /data/iqdoctor/ai-toolkit/.venv/bin/python - <<'PY'
import flux_train_ui
print('import flux_train_ui OK')
PY

This now imports successfully on gradio 6.10.0.

Note: there is still a separate non-blocking Gradio 6 warning about theme / css being passed to Blocks(...); this PR intentionally keeps scope to the startup blocker.