fix: resolve spaced page slugs

[simonbc]

Summary

• Decode page URL slugs back into page names before lookup/save so foo_bar resolves to a page named foo bar.
• Align the sidebar new-page redirect with the server slug format.
• Add regression coverage for spaced page names.

Tests

• .venv/bin/python -m pytest tests/test_urls.py tests/test_page_edit.py