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/basic-toml/src/error.rs

@@ -0,0 +1,54 @@
+use std::fmt::{self, Debug, Display};
+
+/// Errors that can occur when serializing or deserializing TOML.
+pub struct Error(Box<ErrorInner>);
+
+pub(crate) enum ErrorInner {
+    Ser(crate::ser::Error),
+    De(crate::de::Error),
+}
+
+impl Error {
+    /// Produces a (line, column) pair of the position of the error if
+    /// available.
+    ///
+    /// All indexes are 0-based.
+    pub fn line_col(&self) -> Option<(usize, usize)> {
+        match &*self.0 {
+            ErrorInner::Ser(_) => None,
+            ErrorInner::De(error) => error.line_col(),
+        }
+    }
+}
+
+impl From<crate::ser::Error> for Error {
+    fn from(error: crate::ser::Error) -> Self {
+        Error(Box::new(ErrorInner::Ser(error)))
+    }
+}
+
+impl From<crate::de::Error> for Error {
+    fn from(error: crate::de::Error) -> Self {
+        Error(Box::new(ErrorInner::De(error)))
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match &*self.0 {
+            ErrorInner::Ser(error) => Display::fmt(error, formatter),
+            ErrorInner::De(error) => Display::fmt(error, formatter),
+        }
+    }
+}
+
+impl Debug for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match &*self.0 {
+            ErrorInner::Ser(error) => Debug::fmt(error, formatter),
+            ErrorInner::De(error) => Debug::fmt(error, formatter),
+        }
+    }
+}
+
+impl std::error::Error for Error {}