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-hal-nb/README.md

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+[![crates.io](https://img.shields.io/crates/d/embedded-hal-nb.svg)](https://crates.io/crates/embedded-hal-nb)
+[![crates.io](https://img.shields.io/crates/v/embedded-hal-nb.svg)](https://crates.io/crates/embedded-hal-nb)
+[![Documentation](https://docs.rs/embedded-hal-nb/badge.svg)](https://docs.rs/embedded-hal-nb)
+![Minimum Supported Rust Version](https://img.shields.io/badge/rustc-1.60+-blue.svg)
+
+# `embedded-hal-nb`
+
+A non-blocking Hardware Abstraction Layer (HAL) for embedded systems, using the `nb` crate.
+
+This crate contains versions of some [`embedded-hal`](https://crates.io/crates/embedded-hal) traits using `nb`, and shares its scope and [design goals](https://docs.rs/embedded-hal/latest/embedded_hal/#design-goals).
+
+This project is developed and maintained by the [HAL team](https://github.com/rust-embedded/wg#the-hal-team).
+
+## [API reference]
+
+[API reference]: https://docs.rs/embedded-hal-nb
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.60 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
+See [here](../docs/msrv.md) for details on how the MSRV may be upgraded.
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.