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/ssd1306/src/rotation.rs

@@ -0,0 +1,14 @@
+//! Display rotation.
+
+/// Display rotation.
+#[derive(Copy, Clone, Debug)]
+pub enum DisplayRotation {
+    /// No rotation, normal display
+    Rotate0,
+    /// Rotate by 90 degrees clockwise
+    Rotate90,
+    /// Rotate by 180 degrees clockwise
+    Rotate180,
+    /// Rotate 270 degrees clockwise
+    Rotate270,
+}