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/font.rs

@@ -0,0 +1,36 @@
+use embedded_graphics::{
+    image::ImageRaw,
+    mock_display::MockDisplay,
+    mono_font::{mapping::StrGlyphMapping, DecorationDimensions, MonoFont, MonoTextStyle},
+    pixelcolor::BinaryColor,
+    prelude::*,
+    text::{Baseline, Text},
+};
+
+const DATA: &[u8] = &[0xAA, 0x55];
+
+#[test]
+fn custom_font() {
+    let mapping = StrGlyphMapping::new("01", 0);
+
+    let font = MonoFont {
+        image: ImageRaw::new(DATA, 8),
+        character_size: Size::new(2, 2),
+        character_spacing: 0,
+        baseline: 0,
+        strikethrough: DecorationDimensions::default_strikethrough(2),
+        underline: DecorationDimensions::default_underline(2),
+        glyph_mapping: &mapping,
+    };
+
+    let character_style = MonoTextStyle::new(&font, BinaryColor::On);
+
+    let mut display = MockDisplay::new();
+    Text::with_baseline("01", Point::zero(), character_style, Baseline::Top)
+        .draw(&mut display)
+        .unwrap();
+    display.assert_pattern(&[
+        "# # ", //
+        " # #", //
+    ]);
+}