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-async/README.md

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+[![crates.io](https://img.shields.io/crates/d/embedded-hal-async.svg)](https://crates.io/crates/embedded-hal-async)
+[![crates.io](https://img.shields.io/crates/v/embedded-hal-async.svg)](https://crates.io/crates/embedded-hal-async)
+[![Documentation](https://docs.rs/embedded-hal-async/badge.svg)](https://docs.rs/embedded-hal-async)
+
+# `embedded-hal-async`
+
+An asynchronous Hardware Abstraction Layer (HAL) for embedded systems.
+
+This crate contains asynchronous versions of the [`embedded-hal`](https://crates.io/crates/embedded-hal) traits 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).
+
+## Serial/UART traits
+
+There is no serial traits in `embedded-hal-async`. Instead, use [`embedded-io-async`](https://crates.io/crates/embedded-io-async).
+A serial port is essentially a byte-oriented stream, and that's what `embedded-io-async` models. Sharing the traits
+with all byte streams has some advantages. For example, it allows generic code providing a command-line interface
+or a console to operate either on hardware serial ports or on virtual ones like Telnet or USB CDC-ACM.
+
+## Optional Cargo features
+
+- **`defmt-03`**: Derive `defmt::Format` from `defmt` 0.3 for enums and structs.
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.75 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.