refactor(cli): modularize and harden llmdoc viewer

[pDJJq]

Summary

• split the Viewer backend into lifecycle, HTTP routing, typed state projection, and shared repository-health modules
• split the browser Viewer into native model, graph, detail, app, and CSS assets
• fix edge precedence, metadata-only health reporting, listener accumulation, and quadratic layout behavior
• add responsive and keyboard interactions plus CSP, route whitelisting, canonical document lookup, and sanitized Markdown rendering
• update llmdoc runtime knowledge and scoped fingerprints

Verification

• npm test (53 tests)
• npm run typecheck
• npm run lint
• npm pack --dry-run
• real-browser desktop and narrow-screen QA with 0 console errors/warnings