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/ufmt-write/src/lib.rs

@@ -0,0 +1,48 @@
+//! `μfmt`'s `uWrite` trait
+
+#![cfg_attr(not(feature = "std"), no_std)]
+#![deny(missing_docs)]
+#![deny(rust_2018_compatibility)]
+#![deny(rust_2018_idioms)]
+#![deny(warnings)]
+
+#[cfg(feature = "std")]
+use core::convert::Infallible;
+
+#[allow(deprecated)]
+unsafe fn uninitialized<T>() -> T {
+    core::mem::uninitialized()
+}
+
+/// A collection of methods that are required / used to format a message into a stream.
+#[allow(non_camel_case_types)]
+pub trait uWrite {
+    /// The error associated to this writer
+    type Error;
+
+    /// Writes a string slice into this writer, returning whether the write succeeded.
+    ///
+    /// This method can only succeed if the entire string slice was successfully written, and this
+    /// method will not return until all data has been written or an error occurs.
+    fn write_str(&mut self, s: &str) -> Result<(), Self::Error>;
+
+    /// Writes a [`char`] into this writer, returning whether the write succeeded.
+    ///
+    /// A single [`char`] may be encoded as more than one byte. This method can only succeed if the
+    /// entire byte sequence was successfully written, and this method will not return until all
+    /// data has been written or an error occurs.
+    fn write_char(&mut self, c: char) -> Result<(), Self::Error> {
+        let mut buf: [u8; 4] = unsafe { uninitialized() };
+        self.write_str(c.encode_utf8(&mut buf))
+    }
+}
+
+#[cfg(feature = "std")]
+impl uWrite for String {
+    type Error = Infallible;
+
+    fn write_str(&mut self, s: &str) -> Result<(), Infallible> {
+        self.push_str(s);
+        Ok(())
+    }
+}