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/winnow/examples/custom_error.rs

@@ -0,0 +1,69 @@
+use winnow::error::AddContext;
+use winnow::error::ErrMode;
+use winnow::error::FromExternalError;
+use winnow::error::ParserError;
+use winnow::prelude::*;
+use winnow::stream::Stream;
+
+#[derive(Debug)]
+pub enum CustomError<I> {
+    MyError,
+    Winnow(I),
+    External {
+        cause: Box<dyn std::error::Error + Send + Sync + 'static>,
+        input: I,
+    },
+}
+
+impl<I: Stream + Clone> ParserError<I> for CustomError<I> {
+    type Inner = Self;
+
+    fn from_input(input: &I) -> Self {
+        CustomError::Winnow(input.clone())
+    }
+
+    fn into_inner(self) -> Result<Self::Inner, Self> {
+        Ok(self)
+    }
+}
+
+impl<C, I: Stream> AddContext<I, C> for CustomError<I> {
+    #[inline]
+    fn add_context(
+        self,
+        _input: &I,
+        _token_start: &<I as Stream>::Checkpoint,
+        _context: C,
+    ) -> Self {
+        self
+    }
+}
+
+impl<I: Stream + Clone, E: std::error::Error + Send + Sync + 'static> FromExternalError<I, E>
+    for CustomError<I>
+{
+    #[inline]
+    fn from_external_error(input: &I, e: E) -> Self {
+        CustomError::External {
+            cause: Box::new(e),
+            input: input.clone(),
+        }
+    }
+}
+
+pub fn parse<'s>(_input: &mut &'s str) -> ModalResult<&'s str, CustomError<&'s str>> {
+    Err(ErrMode::Backtrack(CustomError::MyError))
+}
+
+fn main() {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let err = parse.parse_next(&mut "").unwrap_err();
+        assert!(matches!(err, ErrMode::Backtrack(CustomError::MyError)));
+    }
+}