fix(webui-docker): include workspace root node_modules so pnpm symlinks resolve ...

fix(webui-docker): include workspace root node_modules so pnpm symlinks resolve (#1806)

ui/web/node_modules is a pnpm workspace package whose entries are
symlinks into ../../../node_modules/.pnpm/..., i.e. the workspace
root's store. The previous Dockerfile only copied ui/web/node_modules
into the final image, leaving every symlink dangling and breaking
`node start.js` with "Cannot find package 'finalhandler'".

Copy the workspace root node_modules to /node_modules in the final
image so the relative symlinks resolve.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>