Add injectable public base path for hosted wrappers

[benvinegar]

Summary

Adds optional public base-path support so hosted wrappers can mount sideshow under a prefix such as /u/:account while preserving the core app’s internal route shape.

When basePath is unset, self-hosted behavior is unchanged.

What changed

• Adds createApp({ basePath }), accepting a string or request-aware hook.
• Injects window.__SIDESHOW_BASE_PATH__ into the viewer.
• Viewer API calls, iframe URLs, copied/opened surface links, and HTTP MCP returned URLs use the configured base path.
• Keeps /s/:id?part=N as the html-part iframe endpoint.
• In hosted/basePath mode, /s/:id without part redirects to BASE_PATH/?surface=:id so markdown/diff/etc. surfaces open in the full viewer.
• Leaves default self-hosted /s/:id behavior unchanged when basePath is unset.

Validation

npm run typecheck
npm run build

Also validated from the hosted wrapper repo:

npm run typecheck
npm test
npm run build

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