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/usbd-hid/README.md

@@ -0,0 +1,11 @@
+# usbd-hid
+
+[![Crates.io](https://img.shields.io/crates/v/usbd-hid.svg)](https://crates.io/crates/usbd-hid) [![usbd-hid](https://docs.rs/usbd-hid/badge.svg)](https://docs.rs/usbd-hid)
+
+USB HID implementation for [usb-device](https://crates.io/crates/usb-device).
+
+Also implements a cheeky procedural macro for generating HID descriptors.
+
+Examples:
+
+  - [twitching usb mouse](https://github.com/atsamd-rs/atsamd/blob/master/boards/itsybitsy_m0/examples/twitching_usb_mouse.rs)