feat: add MBE health check proxy endpoint

- Added /api/v1/mbe/health proxy in Studio proxy (port 8888)
- Frontend now uses proxy endpoint instead of direct mbeUrl
- Works for both localhost and external access (once proxy is deployed)
- MBE health response: {status: 'healthy', model: 'e4b', layers: 42}

src/views/AdminView.vue

@@ -879,9 +879,9 @@ async function testMbe() {
   testingMbe.value = true
   mbeInfo.value = null
   try {
-    // MarkBaseServer only exposes /health (returns plain text "OK")
-    // Always use /health regardless of hostname
-    const healthUrl = `${mbeUrl.value.replace(/\/$/, '')}/health`
+    // Use Studio proxy to reach MBE (works both locally and remotely)
+    const base = getApiBase()
+    const healthUrl = `${base.replace(/\/$/, '')}/api/v1/mbe/health`
     const resp = await fetch(healthUrl, { signal: AbortSignal.timeout(5000) })
     if (!resp.ok) throw new Error(`HTTP ${resp.status}`)