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/thiserror/src/provide.rs

@@ -0,0 +1,20 @@
+use core::error::{Error, Request};
+
+#[doc(hidden)]
+pub trait ThiserrorProvide: Sealed {
+    fn thiserror_provide<'a>(&'a self, request: &mut Request<'a>);
+}
+
+impl<T> ThiserrorProvide for T
+where
+    T: Error + ?Sized,
+{
+    #[inline]
+    fn thiserror_provide<'a>(&'a self, request: &mut Request<'a>) {
+        self.provide(request);
+    }
+}
+
+#[doc(hidden)]
+pub trait Sealed {}
+impl<T: Error + ?Sized> Sealed for T {}