Isolate docs bundles and route-specific styles

[zahlekhan]

Summary

• replace React UI barrel imports with explicit component subpath imports across docs, demos, chat, compare, and design-system examples
• dynamically load expensive charts, renderer previews, OSS agent surfaces, and demo preview panels
• move React UI, Thesys, and Fumadocs preset CSS from the global route into the layouts that consume it
• replace the global Fumadocs theme provider with a lightweight site theme provider and keep Fumadocs scoped to docs routes
• remove the root Geist Mono font from unrelated routes
• enable normal production mangling and browser source maps

Why

The shared module and stylesheet graph caused /cloud to retain UI components, charts, GenUI libraries, agent surfaces, and route-specific CSS that it did not use. Importing package barrels also limited useful tree-shaking and chunk isolation.

Impact

This is the bundle/style-boundary slice of #879. It contains many files, but most changes are mechanical import-path replacements. The combined optimized build reduced total transfer from 3.9 MB to 514 KiB and Total Blocking Time from 830 ms to 10 ms.

Validation

• pnpm --dir docs types:check
• staged diff integrity check
• the focused ESLint run reaches three existing set-state-in-effect errors in GitHub demo state initialization and the demos theme listener; this PR only changes imports/dynamic boundaries in those files

Tradeoffs

• dynamically loaded previews may briefly show their existing loading state
• production browser source maps increase deployment artifact size and may be publicly served depending on hosting configuration

Supersedes the bundle and style isolation portion of #879.