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/embedded-hal-0.2.7/src/watchdog.rs

@@ -0,0 +1,30 @@
+//! Traits for interactions with a processors watchdog timer.
+
+/// Feeds an existing watchdog to ensure the processor isn't reset. Sometimes
+/// commonly referred to as "kicking" or "refreshing".
+#[cfg(feature = "unproven")]
+pub trait Watchdog {
+    /// Triggers the watchdog. This must be done once the watchdog is started
+    /// to prevent the processor being reset.
+    fn feed(&mut self);
+}
+
+/// Enables A watchdog timer to reset the processor if software is frozen or
+/// stalled.
+#[cfg(feature = "unproven")]
+pub trait WatchdogEnable {
+    /// Unit of time used by the watchdog
+    type Time;
+    /// Starts the watchdog with a given period, typically once this is done
+    /// the watchdog needs to be kicked periodically or the processor is reset.
+    fn start<T>(&mut self, period: T)
+    where
+        T: Into<Self::Time>;
+}
+
+/// Disables a running watchdog timer so the processor won't be reset.
+#[cfg(feature = "unproven")]
+pub trait WatchdogDisable {
+    /// Disables the watchdog
+    fn disable(&mut self);
+}