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/esp-synopsys-usb-otg/build.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let profile = std::env::var("PROFILE").unwrap();
+    let target = std::env::var("TARGET").unwrap();
+
+    if profile == "debug" && !target.starts_with("x86_64") {
+        println!("cargo:warning=synopsys-usb-otg is being compiled in debug mode. This driver works reliably only in release mode!");
+    }
+}