Add Agents UI page + installer script endpoint

- client/src/Agents.js: page showing registered Linux agents with live
  status, CPU/memory/disk gauge bars, load/uptime/network stats, and
  an install-instructions panel with copyable curl commands
- client/src/Agents.css: dark-theme card grid styles matching the app
- client/src/App.js: adds /agents route and Agents nav link
- server/index.js: adds GET /api/agents/installer to serve install-agent.sh
- client/build: production build updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
-    "main.css": "/static/css/main.d4111563.css",
-    "main.js": "/static/js/main.ee340b3d.js",
+    "main.css": "/static/css/main.e143c118.css",
+    "main.js": "/static/js/main.a720143a.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
-    "main.d4111563.css.map": "/static/css/main.d4111563.css.map",
-    "main.ee340b3d.js.map": "/static/js/main.ee340b3d.js.map",
+    "main.e143c118.css.map": "/static/css/main.e143c118.css.map",
+    "main.a720143a.js.map": "/static/js/main.a720143a.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
-    "static/css/main.d4111563.css",
-    "static/js/main.ee340b3d.js"
+    "static/css/main.e143c118.css",
+    "static/js/main.a720143a.js"
   ]
 }