fix(ui): install chat modules from released archives

[vernonstinebaker]

Summary

• resolve actual UI module release assets from GitHub release metadata instead of assuming only *-bundle.tar.gz
• accept the current nullclaw-chat-ui-<tag>.tar.gz archive naming used by nullclaw-chat-ui releases
• flatten extracted archives when they contain a single top-level directory so installed files land at the module root where NullHub expects module.js
• add focused tests for asset selection and extracted-layout normalization

Why

NullHub's chat tab dynamically imports /ui/<module>@<version>/module.js. Recent nullclaw-chat-ui releases published archives containing a top-level package directory and no *-bundle.tar.gz asset, so NullHub could install a module directory but still fail to serve the embeddable entrypoint where the UI expected it. That left the chat tab stuck on Failed to load module: Importing a module script failed.

Validation

• zig build test -Dbuild-ui=false -Dembed-ui=false --summary all
• local live smoke after rebuild/restart:
  • GET /api/ui-modules
  • GET /ui/nullclaw-chat-ui@latest/module.js -> 200 OK
  • confirmed chat tabs for default and willy-nilly load once the fixed chat-ui release layout is installed

Notes

• this is the NullHub-side compatibility half of the chat fix
• the corresponding nullclaw-chat-ui release packaging fix is submitted separately