webclient: artifacts panel — compact actions, refresh in static row, file icons via phosphor 2.x ...

- artifact-actions: flex wrap gap 4px instead of full-width stretched grid
- files refresh: static toolbar row (GnIconButton sm) instead of floating
  absolute button that sat on top of the first list rows
- file icons: phosphor vendored 1.4.2 lacked the file-type glyphs
  (py/ts/js/html/css...) that fileIcon() already mapped — upgrade to
  @phosphor-icons/web 2.1.2 (bundled regular font), drop the vendor dir;
  json -> file-code (phosphor has no json glyph)

Migrate to Vue webclient; rename old client to old_webclient ...

- client/ → old_webclient/ (vanilla JS client preserved as reference)
- webclient/ — new Vue 3 + Pinia webclient (source + dist build)
- vite.config.js: outDir changed to webclient/dist/
- main.py: serve /assets and /images from webclient/dist/,
  index.html from webclient/dist/index.html
- .gitignore: exclude webclient/node_modules/, include webclient/dist/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>