Add "View source data on GitHub" button to site header

[Copilot]

The GitHub Pages publication site had no link back to the source repository.

Changes

• New --repo-url CLI arg (csv_to_html.py): optional; when provided, renders a "View source data on GitHub" anchor in the page header. Omitting it hides the button entirely (backward compatible).
• GitHub icon + styling: inline SVG octicon, .gh-source-btn CSS class matching the existing button style, placed as a flex sibling of the title block in the header (justify-content: space-between pushes it to the top-right).
• Workflow updated: deploy-pages.yml passes --repo-url "https://github.com/avc-adelaide/wspr-publications".

- name: Build static site
  run: |
    python scripts/csv_to_html.py \
      --title "WSPR Publications" \
      --scholar-user-id kxCnpPEAAAAJ \
      --repo-url "https://github.com/avc-adelaide/wspr-publications" \
      ...

URL is HTML-escaped before insertion into the href attribute.

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