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/futures-util/src/future/option.rs

@@ -0,0 +1,64 @@
+//! Definition of the `Option` (optional step) combinator
+
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// A future representing a value which may or may not be present.
+    ///
+    /// Created by the [`From`] implementation for [`Option`](std::option::Option).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::future::OptionFuture;
+    ///
+    /// let mut a: OptionFuture<_> = Some(async { 123 }).into();
+    /// assert_eq!(a.await, Some(123));
+    ///
+    /// a = None.into();
+    /// assert_eq!(a.await, None);
+    /// # });
+    /// ```
+    #[derive(Debug, Clone)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct OptionFuture<F> {
+        #[pin]
+        inner: Option<F>,
+    }
+}
+
+impl<F> Default for OptionFuture<F> {
+    fn default() -> Self {
+        Self { inner: None }
+    }
+}
+
+impl<F: Future> Future for OptionFuture<F> {
+    type Output = Option<F::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.project().inner.as_pin_mut() {
+            Some(x) => x.poll(cx).map(Some),
+            None => Poll::Ready(None),
+        }
+    }
+}
+
+impl<F: FusedFuture> FusedFuture for OptionFuture<F> {
+    fn is_terminated(&self) -> bool {
+        match &self.inner {
+            Some(x) => x.is_terminated(),
+            None => true,
+        }
+    }
+}
+
+impl<T> From<Option<T>> for OptionFuture<T> {
+    fn from(option: Option<T>) -> Self {
+        Self { inner: option }
+    }
+}