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)