Add custom 404 page as smart router for repo URLs

[jonathanpopham]

Context

GitHub Pages serves 404.html for any unmatched route. We can use this as a smart router so repos.supermodeltools.com/react shows docs and repos.supermodeltools.com/unknown-repo shows a "Generate" landing page.

Requirements

Create 404.html that reads the URL path, checks if docs exist, and either redirects or shows a branded landing page with a Generate button (creates a [Repo Request] issue). Sanitize URL path to prevent XSS. Update generate-index.go to include it. Follow existing visual style. Must be mobile responsive.

Acceptance criteria

• /unknown-repo shows branded landing page with Generate button
• No XSS via URL path manipulation
• Generate button creates a Repo Request issue
• Mobile responsive

@claude

[coderabbitai]

🔗 Related PRs

#5 - Inline submit — no page navigation, everything in background [closed]

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[jonathanpopham]

@claude Please implement this issue. Create a PR with the custom 404 page.

[jonathanpopham]

@claude Please implement this issue.

[claude]

Claude finished @jonathanpopham's task in 5m 12s —— View job • claude/issue-11-20260303-1930 • [Create PR ➔](https://github.com/supermodeltools/supermodeltools.github.io/compare/main...claude/issue-11-20260303-1930?quick_pull=1&title=Issue%20%2311%3A%20Changes%20from%20Claude&body=This%20PR%20addresses%20issue%20%2311%0A%0AGenerated%20with%20%5BClaude%20Code%5D(https%3A%2F%2Fclaude.ai%2Fcode)

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Implementing Issue #11: Custom 404 Smart Router

• Gather context
• Understand existing code structure
• Create 404.html with smart routing logic
• XSS prevention for URL paths
• Generate button for Repo Request issues
• Mobile responsive design
• Update generate-index.go to output 404.html
• Commit and push changes
• Create PR

What was implemented

Added a generate404() function to generate-index.go that outputs site/404.html on each build. The page embeds the full list of known repos as a JSON array at build time, then uses JavaScript to:

1. Smart redirect: if the URL path (e.g. /react) matches a known repo, immediately redirect to /{repo}/ (the docs URL)
2. Generate landing page: if unknown, show a branded page with a Generate Docs button that opens a pre-filled GitHub issue: [Repo Request] {slug} with @claude in the body to auto-trigger

XSS prevention:

• textContent used for all user-controlled display (never innerHTML)
• encodeURIComponent() used when building issue URL parameters
• No innerHTML usage anywhere in the 404 page

Other details:

• Mobile responsive with @media (max-width: 768px) breakpoints
• Matches existing visual style and CSS variables
• <noscript> fallback shows content without JavaScript

View PR #12
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