fix: add close button to about overlay

[1cbyc]

Summary

• Add an explicit close button to the About overlay
• Keep the existing history/back button behavior intact
• Cover the generated About HTML in the processor tests

Test Plan

• cd scraper && .venv/bin/python -m pytest -q tests/test_processor.py -k 'about_html'
• cd scraper && .venv/bin/ruff check src/maps2zim/processor.py tests/test_processor.py
• cd scraper && .venv/bin/ruff format --check src/maps2zim/processor.py tests/test_processor.py
• git diff --check

Fixes #134

[benoit74]

I need more input from @kelson42

The "about" is not an overlay/popup but a dedicated page (which might look like an overlay/popup, I get it). So the "close button" implemented in this PR is a trick which navigates back to the home page. This is a trick, but since this is the only page the user might come from, it works. Is this what you expected or would you prefer we move the about to a real popup, since this also has the advantage that we do not have to decide about a background color anymore? I feel like this would make more sense