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.

crates/isokit-wasm/src/lib.rs

@@ -216,6 +216,7 @@ mod tests {
         assert_eq!(m["title"], "isokit demo works");
         assert!(m["catalog"].as_array().unwrap().len() > 20);
         assert!(m["script_error"].is_null());
+        assert_eq!(m["engine"], env!("CARGO_PKG_VERSION"));
         let n = j(&s.state())["doc"]["objects"].as_array().unwrap().len();
         let v = j(&s.render("site", 1, "", true));
         assert!(