`View Source` toolbar button renders `href="None"` on auto-generated pages (genindex, search, proof index)

Summary

On Sphinx auto-generated pages (genindex.html, search.html, and the proof-domain index prf-prf.html), the "View Source" GitHub button in the toolbar renders as <a href="None" ...>, producing broken links. Regular lecture pages render correctly.

Reproducer

Build any project using this theme that has a repository_url configured and at least one prf:proof directive. Inspect the generated _build/html/genindex.html, _build/html/search.html, and _build/html/prf-prf.html — the toolbar's GitHub icon <a> has href="None".

Caught by lychee link-checker on lecture-python.myst:

```

Errors in genindex.html

• [ERROR] file:///.../None | Cannot find file: File not found.

Errors in search.html

• [ERROR] file:///.../None | Cannot find file: File not found.

Errors in prf-prf.html

• [ERROR] file:///.../None | Cannot find file: File not found.
  ```

Root cause

In `src/quantecon_book_theme/init.py`, `theme_repository_url` is only populated inside the `if doctree and hasattr(app.env, "doc2path"):` branch (lines ~470–472). On auto-generated pages there's no source doctree, so the `else` branches (lines 481 and 487) set `context["theme_repository_url"] = None`.

`src/quantecon_book_theme/theme/quantecon_book_theme/layout.html:399` then interpolates the value unguarded:

```jinja

...

\`\`\`

Jinja stringifies Python `None` to the literal `"None"`, producing `href="None"`.

Suggested fix

Two reasonable options (either or both):

1. Guard the template — match the existing `{%- if notebook_path %}` / `{%- if theme_nb_repository_url %}` pattern used a few lines above:
   ```jinja
   {%- if theme_repository_url %}
2. ...
{%- endif %} \`\`\`
3. Populate the variable unconditionally in `init.py` — read `repository_url` from `config_theme` regardless of whether a doctree is present, so the global "View Source" link works on auto-generated pages too.

Option 1 alone removes the broken link; option 1 + 2 also keeps the button visible on auto-generated pages.

Version

Reproduced on `main` (v0.20.3); also affects v0.20.2 (deployed on lecture-python.myst).

Read src/quantecon_book_theme/init.py around theme_repository_url and the View Source block in src/quantecon_book_theme/theme/quantecon_book_theme/layout.html. Build a project with repository_url and a prf:proof directive, then inspect genindex.html, search.html, and prf-prf.html. Done means those pages no longer contain href="None" and the intended View Source behavior is preserved.