ui: add Section 4 to invoke spimpack package from the Streamlit UI

[Copilot]

The Streamlit UI only supported defining the manifest and dataset; there was no way to run spimpack package without leaving the browser.

Changes

ui/app.py

• Added Section 4 – Run spimpack package at the bottom of the page
• Inputs: manifest save path, output directory, backend selector (symlink), relative-symlinks checkbox
• Live command preview updates as fields are filled:

  spimpack package --manifest /data/project/manifest.yml --output-dir /data/bids_output --backend symlink

• "💾 Save Files & ▶️ Run spimpack package" button:
  • Writes manifest.yml + TSV to disk at the specified path
  • Calls run_package() in-process (via contextlib.redirect_stdout) to capture and display output
  • Shows inline success/error feedback with captured stdout
• Button is disabled while validation errors exist or required paths are empty
• Added import contextlib, import io, and imports of run_package / ValidationError from spimpack