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/bitflags-1.3.2/src/example_generated.rs

@@ -0,0 +1,14 @@
+//! This module shows an example of code generated by the macro. **IT MUST NOT BE USED OUTSIDE THIS
+//! CRATE**.
+
+bitflags! {
+    /// This is the same `Flags` struct defined in the [crate level example](../index.html#example).
+    /// Note that this struct is just for documentation purposes only, it must not be used outside
+    /// this crate.
+    pub struct Flags: u32 {
+        const A = 0b00000001;
+        const B = 0b00000010;
+        const C = 0b00000100;
+        const ABC = Self::A.bits | Self::B.bits | Self::C.bits;
+    }
+}