studio: version text in the status bar, favicon

The status bar ends with "isokit <engine version>"; the web build adds
the commit (linked on Gitea), its date as a tooltip and a "+" when built
from a tree with uncommitted changes (build_web.py writes a
<meta name="isokit-build"> from GITHUB_SHA or git). /api/meta now carries
"engine". studio/favicon.svg (isometric cube in the accent green) is
served by `isokit studio` (also on /view) and copied into the web build.
check_web.py and check_studio.py check both.

studio/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <path d="M16 2.5 29 10 16 17.5 3 10Z" fill="#3ddc84"/>
+  <path d="M3 10 16 17.5V30.5L3 23Z" fill="#1f9e5a"/>
+  <path d="M29 10 16 17.5V30.5L29 23Z" fill="#126b3c"/>
+  <path d="M16 2.5 29 10V23L16 30.5 3 23V10ZM3 10 16 17.5 29 10M16 17.5V30.5" fill="none" stroke="#0a0f1a" stroke-width="1.2" stroke-linejoin="round"/>
+</svg>