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_boxed.rs

@@ -0,0 +1,45 @@
+#![allow(
+    // Clippy bug: https://github.com/rust-lang/rust-clippy/issues/7422
+    clippy::nonstandard_macro_braces,
+)]
+
+use anyhow::anyhow;
+use std::error::Error as StdError;
+use std::io;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("outer")]
+struct MyError {
+    source: io::Error,
+}
+
+#[test]
+fn test_boxed_str() {
+    let error = Box::<dyn StdError + Send + Sync>::from("oh no!");
+    let error = anyhow!(error);
+    assert_eq!("oh no!", error.to_string());
+    assert_eq!(
+        "oh no!",
+        error
+            .downcast_ref::<Box<dyn StdError + Send + Sync>>()
+            .unwrap()
+            .to_string()
+    );
+}
+
+#[test]
+fn test_boxed_thiserror() {
+    let error = MyError {
+        source: io::Error::new(io::ErrorKind::Other, "oh no!"),
+    };
+    let error = anyhow!(error);
+    assert_eq!("oh no!", error.source().unwrap().to_string());
+}
+
+#[test]
+fn test_boxed_anyhow() {
+    let error = anyhow!("oh no!").context("it failed");
+    let error = anyhow!(error);
+    assert_eq!("oh no!", error.source().unwrap().to_string());
+}