feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/anyhow/tests/test_backtrace.rs

@@ -0,0 +1,15 @@
+#![allow(clippy::let_underscore_untyped)]
+
+#[rustversion::not(nightly)]
+#[ignore = "requires nightly"]
+#[test]
+fn test_backtrace() {}
+
+#[rustversion::nightly]
+#[test]
+fn test_backtrace() {
+    use anyhow::anyhow;
+
+    let error = anyhow!("oh no!");
+    let _ = error.backtrace();
+}