docs(roadmap): add egui-rewrite update notes, document npx exception

Several shipped-feature entries (themes, process panel, battery panel,
floating panel layout) still described their original Tauri/WebView2
implementation with no note that the app has since moved to egui, unlike
the Settings redesign entry which already had one -- add the same kind of
update note to each.

Also clarify that "Remove Node.js / npm infrastructure" has one intentional
exception: release.yml still runs npx for a legacy Tauri minisign signature
so pre-1.26 clients can update without crashing. Document that step, the
resulting latest.json signature field, and the previously-undocumented
winget-submit.yml workflow in docs/release.md.

Closes #164