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/studio.css

@@ -52,6 +52,8 @@ input[type=checkbox] { accent-color: var(--accent); }
 .st-hint { position: absolute; top: 52px; left: 50%; transform: translateX(-50%); background: var(--panel); border: 1px solid var(--accent); border-radius: 16px; padding: 4px 14px; z-index: 5; pointer-events: none; }
 .st-status { display: flex; align-items: center; gap: 14px; padding: 4px 10px; background: var(--panel); border-top: 1px solid var(--stroke); color: var(--mute); font-variant-numeric: tabular-nums; flex: 0 0 auto; }
 .st-status select { padding: 1px 4px; }
+.st-ver { color: var(--mute); font-size: 11px; text-decoration: none; white-space: nowrap; }
+.st-ver:hover { color: var(--fg); text-decoration: underline; }
 .st-link { background: none; border: 0; color: var(--danger); padding: 0; text-decoration: underline; }
 
 .st-right h3 { margin: 0; font-size: 15px; display: flex; align-items: center; gap: 6px; }