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-graphics/tests/custom_primitive_style.rs

@@ -0,0 +1,49 @@
+use embedded_graphics::{
+    mock_display::MockDisplay,
+    pixelcolor::Rgb888,
+    prelude::*,
+    primitives::{Rectangle, StyledDrawable},
+};
+
+struct CheckerboardStyle<C>(C, C);
+
+impl<C: PixelColor> StyledDrawable<CheckerboardStyle<C>> for Rectangle {
+    type Color = C;
+    type Output = ();
+
+    fn draw_styled<D>(
+        &self,
+        style: &CheckerboardStyle<C>,
+        target: &mut D,
+    ) -> Result<Self::Output, D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>,
+    {
+        target.fill_contiguous(
+            self,
+            self.points().map(|p| {
+                if (p.x % 2 == 0) ^ (p.y % 2 == 0) {
+                    style.1
+                } else {
+                    style.0
+                }
+            }),
+        )
+    }
+}
+
+#[test]
+fn custom_primitive_style() {
+    let style = CheckerboardStyle(Rgb888::RED, Rgb888::GREEN);
+
+    let mut display = MockDisplay::new();
+    Rectangle::new(Point::zero(), Size::new(4, 3))
+        .into_styled(style)
+        .draw(&mut display)
+        .unwrap();
+    display.assert_pattern(&[
+        "RGRG", //
+        "GRGR", //
+        "RGRG", //
+    ]);
+}