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/display.rs

@@ -0,0 +1,82 @@
+use core::fmt::Display;
+#[cfg(feature = "std")]
+use std::path::{self, Path, PathBuf};
+
+#[doc(hidden)]
+pub trait AsDisplay<'a>: Sealed {
+    // TODO: convert to generic associated type.
+    // https://github.com/dtolnay/thiserror/pull/253
+    type Target: Display;
+
+    fn as_display(&'a self) -> Self::Target;
+}
+
+impl<'a, T> AsDisplay<'a> for &T
+where
+    T: Display + ?Sized + 'a,
+{
+    type Target = &'a T;
+
+    fn as_display(&'a self) -> Self::Target {
+        *self
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'a> AsDisplay<'a> for Path {
+    type Target = path::Display<'a>;
+
+    #[inline]
+    fn as_display(&'a self) -> Self::Target {
+        self.display()
+    }
+}
+
+#[cfg(feature = "std")]
+impl<'a> AsDisplay<'a> for PathBuf {
+    type Target = path::Display<'a>;
+
+    #[inline]
+    fn as_display(&'a self) -> Self::Target {
+        self.display()
+    }
+}
+
+#[doc(hidden)]
+pub trait Sealed {}
+impl<T: Display + ?Sized> Sealed for &T {}
+#[cfg(feature = "std")]
+impl Sealed for Path {}
+#[cfg(feature = "std")]
+impl Sealed for PathBuf {}
+
+// Add a synthetic second impl of AsDisplay to prevent the "single applicable
+// impl" rule from making too weird inference decision based on the single impl
+// for &T, which could lead to code that compiles with thiserror's std feature
+// off but breaks under feature unification when std is turned on by an
+// unrelated crate.
+#[cfg(not(feature = "std"))]
+mod placeholder {
+    use super::{AsDisplay, Sealed};
+    use core::fmt::{self, Display};
+
+    #[allow(dead_code)]
+    pub struct Placeholder;
+
+    impl<'a> AsDisplay<'a> for Placeholder {
+        type Target = Self;
+
+        #[inline]
+        fn as_display(&'a self) -> Self::Target {
+            Placeholder
+        }
+    }
+
+    impl Display for Placeholder {
+        fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+            unreachable!()
+        }
+    }
+
+    impl Sealed for Placeholder {}
+}