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/basic-toml/README.md

@@ -0,0 +1,36 @@
+# basic-toml
+
+[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/basic--toml-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/basic-toml)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/basic-toml.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/basic-toml)
+[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-basic--toml-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/basic-toml)
+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/basic-toml/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/basic-toml/actions?query=branch%3Amaster)
+
+A library for parsing and producing data in [TOML] format using [Serde].
+
+This crate is a stripped down fork of version 0.5 of the `toml` crate (from
+before the `toml_edit` rewrite).
+
+[TOML]: https://toml.io
+[Serde]: https://serde.rs
+
+```toml
+[dependencies]
+basic-toml = "0.1"
+```
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.
+</sub>