Reduce CPU usage with conditional rendering

Implement event-driven rendering to dramatically reduce CPU usage.
Only render when something actually changes instead of constantly
rendering at 20 FPS.

Changes:
- Increase poll timeout from 50ms to 200ms (5 FPS)
- Add needs_render flag to track when rendering is required
- Trigger rendering only on: user input, new metrics, heartbeat
  checks, or terminal resize events
- Reset render flag after each render cycle

Based on cm-player optimization approach.

dashboard/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cm-dashboard"
-version = "0.1.265"
+version = "0.1.266"
 edition = "2021"
 
 [dependencies]