Add index landing page for relay server

- Add index.html explaining what MacroPad Relay is
- Add /ping endpoint for container health checks
- Add route for index page

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

macropad-relay/src/server.ts

@@ -55,6 +55,16 @@ export function createServer() {
     });
   });
 
+  // Ping endpoint for container health checks
+  app.get('/ping', (req, res) => {
+    res.json({ status: 'ok' });
+  });
+
+  // Index page
+  app.get('/', (req, res) => {
+    res.sendFile(path.join(publicPath, 'index.html'));
+  });
+
   // Login page for session
   app.get('/:sessionId', (req, res) => {
     const session = sessionManager.getSession(req.params.sessionId);