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/quote/src/spanned.rs

@@ -0,0 +1,49 @@
+use crate::ToTokens;
+use proc_macro2::extra::DelimSpan;
+use proc_macro2::{Span, TokenStream};
+
+// Not public API other than via the syn crate. Use syn::spanned::Spanned.
+pub trait Spanned: private::Sealed {
+    fn __span(&self) -> Span;
+}
+
+impl Spanned for Span {
+    fn __span(&self) -> Span {
+        *self
+    }
+}
+
+impl Spanned for DelimSpan {
+    fn __span(&self) -> Span {
+        self.join()
+    }
+}
+
+impl<T: ?Sized + ToTokens> Spanned for T {
+    fn __span(&self) -> Span {
+        join_spans(self.into_token_stream())
+    }
+}
+
+fn join_spans(tokens: TokenStream) -> Span {
+    let mut iter = tokens.into_iter().map(|tt| tt.span());
+
+    let Some(first) = iter.next() else {
+        return Span::call_site();
+    };
+
+    iter.fold(None, |_prev, next| Some(next))
+        .and_then(|last| first.join(last))
+        .unwrap_or(first)
+}
+
+mod private {
+    use crate::ToTokens;
+    use proc_macro2::extra::DelimSpan;
+    use proc_macro2::Span;
+
+    pub trait Sealed {}
+    impl Sealed for Span {}
+    impl Sealed for DelimSpan {}
+    impl<T: ?Sized + ToTokens> Sealed for T {}
+}