chore(ui): migrate vite 6 -> 8 and plugin-react 4 -> 6

Supersedes the auto-generated bump in #2496, which only updated vite and
left @vitejs/plugin-react on a peer range that excludes vite 8, breaking
the UI build (and every Go job that embeds the UI assets) with ERESOLVE.

- vite ^6.0.0 -> ^8.0.13
- @vitejs/plugin-react ^4.3.0 -> ^6.0.2 (peers vite ^8.0.0 only)
- vite-plugin-singlefile ^2.0.0 -> ^2.3.3 (peers already allowed v8)
- engines.node >=20 -> ^20.19.0 || >=22.12.0 (Vite 7+ requirement)

Vite 8 ships Rolldown instead of Rollup, which rejects bundle mutation in
generateBundle. The rename-output plugin was doing exactly that to flatten
the singlefile-inlined HTML from src/apps/<app>/index.html down to
<app>.html. Refactored it to hoist the file in closeBundle (post-write)
and renamed it to flatten-output to reflect what it actually does.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums