ui: serve static assets on Windows

[aeron-gh]

On Windows the web UI fails to load — every /static/* request (the React JS/CSS bundles) returns 404, so you end up with a blank page.

The /static/*filepath handler builds the asset path with filepath.Join, which uses the OS separator (\ on Windows). The assets are served from an embed.FS, and io/fs paths are always forward-slash separated regardless of OS, so the lookup never matches and the asset 404s. path.Join keeps the path slash-separated on every platform.

Same filepath.Join → path.Join class as #8361, just in a different spot (that one is about object-storage keys, this is the embedded UI).

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• pkg/ui/ui.go: use path.Join instead of filepath.Join when resolving embedded static assets.

Verification

Added TestServeStaticAsset, which requests a real embedded asset through the /static/*filepath route and asserts it returns 200 with the file contents. It fails (404) with filepath.Join on Windows and passes with path.Join.

[aeron-gh]

the failing Doc check is unrelated to this change. All the code checks pass; a re-run of that job should clear it