feat: integrate reviewed proxy page and managed model keys

Source-candidate: poe d9a564e6d20f5f0fd75ce45b12cbbf8bb4c9d568\nSource-candidate: magica38ba5762b06716db1dee00d0afec162656488f5c\nValidated321whole-file phase plus whole bus1349/model-proxy100/webui490; final gate tooling checks pending, not landed.

scripts/mosaic-config.mjs

@@ -353,7 +353,7 @@ async function validate(document, file) {
       const { loadProxy } = await import("../packages/model-proxy/src/settings.mjs");
       proxy = loadProxy(path.dirname(file), dataRoot);
     }
-  } catch { fail(2, "proxy-config-invalid"); }
+  } catch (error) { fail(2, ['proxy-rotate-grace-invalid', 'proxy-first-output-invalid'].includes(error.code) ? error.code : "proxy-config-invalid"); }
 
   return {
     configVersion: document.configVersion,