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/embassy-usb-driver/README.md

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+# embassy-usb-driver
+
+This crate contains the driver traits for [`embassy-usb`]. HAL/BSP crates can implement these
+traits to add support for using `embassy-usb` for a given chip/platform.
+
+The traits are kept in a separate crate so that breaking changes in the higher-level [`embassy-usb`]
+APIs don't cause a semver-major bump of this crate. This allows existing HALs/BSPs to be used
+with the newer `embassy-usb` without needing updates.
+
+If you're writing an application using USB, you should depend on the main [`embassy-usb`] crate
+instead of this one.
+
+[`embassy-usb`]: https://crates.io/crates/embassy-usb
+
+## Interoperability
+
+This crate can run on any executor.