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>

server/index.js

@@ -2170,7 +2170,7 @@ app.post('/api/resources/import', express.text({ type: 'text/csv' }), (req, res)
 
 // ===== Agent API =====
 
-// Serve the agent install script so remote hosts can curl it
+// Serve the agent daemon script
 app.get('/api/agents/script', (req, res) => {
   const fs = require('fs');
   const scriptPath = path.join(__dirname, '../agent/skywatch-agent.sh');
@@ -2183,6 +2183,19 @@ app.get('/api/agents/script', (req, res) => {
   }
 });
 
+// Serve the installer script
+app.get('/api/agents/installer', (req, res) => {
+  const fs = require('fs');
+  const installerPath = path.join(__dirname, '../agent/install-agent.sh');
+  if (fs.existsSync(installerPath)) {
+    res.setHeader('Content-Type', 'text/x-sh');
+    res.setHeader('Content-Disposition', 'attachment; filename="install-agent.sh"');
+    res.sendFile(installerPath);
+  } else {
+    res.status(404).json({ error: 'Installer script not found on this server' });
+  }
+});
+
 // Register a new Linux agent — returns a bearer token the agent uses for all future calls
 app.post('/api/agents/register', rateLimit(5), (req, res) => {
   // Optional: if AGENT_REGISTRATION_KEY is set in env, require it as X-Registration-Key header