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-cli/src/serve.rs

@@ -13,7 +13,7 @@ use tiny_http::{Header, Method, Request, Response, Server};
 const VIEW_PAGE: &str = r#"<!doctype html><html lang="en"><head><meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1"><title>isokit preview</title>
 <script>(function(){var t=null;try{t=localStorage.getItem("isokit-theme")}catch(e){}if(t!=="light"&&t!=="dark")t=matchMedia("(prefers-color-scheme: light)").matches?"light":"dark";document.documentElement.setAttribute("data-theme",t);})();</script>
-<link rel="stylesheet" href="/assets/viewer/isokit-view.css"><link rel="stylesheet" href="/assets/theme.css">
+<link rel="icon" type="image/svg+xml" href="/assets/studio/favicon.svg"><link rel="stylesheet" href="/assets/viewer/isokit-view.css"><link rel="stylesheet" href="/assets/theme.css">
 <style>:root{color-scheme:dark}html[data-theme=light]{color-scheme:light}body{margin:0;background:#0a0f1a;font:14px system-ui,sans-serif;color:#d6deeb}
 html[data-theme=light] body{background:#f4f6fa;color:#1b2533}main{max-width:1320px;margin:0 auto;padding:16px}</style></head>
 <body><main><div id="view"></div></main><script src="/assets/viewer/isokit-view.js"></script><script>
@@ -164,6 +164,8 @@ pub fn studio(dir: &Path, port: u16, open: bool) -> Result<()> {
                     "text/javascript; charset=utf-8"
                 } else if name.ends_with(".css") {
                     "text/css; charset=utf-8"
+                } else if name.ends_with(".svg") {
+                    "image/svg+xml"
                 } else {
                     "text/html; charset=utf-8"
                 };