Update demo page title and screenshots

[jmnote]

Summary

• centralize the demo wiki page title as PAGE_TITLE
• URL-encode the title used by screenshot routes
• show demo source as wikitext and regenerate screenshots

Tests

• make checks
• bash -n hack/demo/demo.sh hack/local-mathjax.sh
• node --check hack/demo/screenshot.mjs
• parsed every demo settings/examples entry

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Update the standalone screenshot URL default

When screenshot.mjs runs with URL unset, its fallback at hack/demo/screenshot.mjs:37 still opens /index.php/Demo, while up() now seeds only the SimpleMathJax page. The script therefore silently captures MediaWiki's missing-page view instead of the demo; update or remove that fallback when changing PAGE_TITLE.

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