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/text_renderer_generic.rs

@@ -0,0 +1,83 @@
+use embedded_graphics::{
+    mock_display::MockDisplay,
+    pixelcolor::Rgb888,
+    prelude::*,
+    primitives::Rectangle,
+    text::{
+        renderer::{TextMetrics, TextRenderer},
+        Baseline, Text,
+    },
+};
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+struct GenericTextStyle<C>(C);
+
+impl<C: PixelColor> TextRenderer for GenericTextStyle<C> {
+    type Color = C;
+
+    fn draw_string<D>(
+        &self,
+        text: &str,
+        position: Point,
+        baseline: Baseline,
+        target: &mut D,
+    ) -> Result<Point, D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>,
+    {
+        let metrics = self.measure_string(text, position, baseline);
+
+        target.fill_solid(&metrics.bounding_box, self.0)?;
+
+        Ok(metrics.next_position)
+    }
+
+    fn draw_whitespace<D>(
+        &self,
+        _width: u32,
+        _position: Point,
+        _baseline: Baseline,
+        _target: &mut D,
+    ) -> Result<Point, D::Error>
+    where
+        D: DrawTarget<Color = Self::Color>,
+    {
+        todo!()
+    }
+
+    fn measure_string(&self, text: &str, position: Point, _baseline: Baseline) -> TextMetrics {
+        // TODO: use baseline
+
+        let width = text.len() as u32 * 4;
+
+        TextMetrics {
+            bounding_box: Rectangle::new(position, Size::new(width, 4)),
+            next_position: position + Size::new(width, 0),
+        }
+    }
+
+    fn line_height(&self) -> u32 {
+        5
+    }
+}
+
+#[test]
+fn generic_text_renderer() {
+    let mut target = MockDisplay::new();
+
+    Text::new("ab\nc", Point::zero(), GenericTextStyle(Rgb888::RED))
+        .draw(&mut target)
+        .unwrap();
+
+    target.assert_pattern(&[
+        "RRRRRRRR", //
+        "RRRRRRRR", //
+        "RRRRRRRR", //
+        "RRRRRRRR", //
+        "        ", //
+        "RRRR    ", //
+        "RRRR    ", //
+        "RRRR    ", //
+        "RRRR    ", //
+    ]);
+}