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/version_check/README.md

@@ -0,0 +1,80 @@
+# version\_check
+
+[![Build Status](https://github.com/SergioBenitez/version_check/workflows/CI/badge.svg)](https://github.com/SergioBenitez/version_check/actions)
+[![Current Crates.io Version](https://img.shields.io/crates/v/version_check.svg)](https://crates.io/crates/version_check)
+[![rustdocs on docs.rs](https://docs.rs/version_check/badge.svg)](https://docs.rs/version_check)
+
+This tiny crate checks that the running or installed `rustc` meets some version
+requirements. The version is queried by calling the Rust compiler with
+`--version`. The path to the compiler is determined first via the `RUSTC`
+environment variable. If it is not set, then `rustc` is used. If that fails, no
+determination is made, and calls return `None`.
+
+## Usage
+
+Add to your `Cargo.toml` file, typically as a build dependency:
+
+```toml
+[build-dependencies]
+version_check = "0.9"
+```
+
+`version_check` is compatible and compiles with Rust 1.0.0 and beyond.
+
+## Examples
+
+Set a `cfg` flag in `build.rs` if the running compiler was determined to be
+at least version `1.13.0`:
+
+```rust
+extern crate version_check as rustc;
+
+if rustc::is_min_version("1.13.0").unwrap_or(false) {
+    println!("cargo:rustc-cfg=question_mark_operator");
+}
+```
+
+Check that the running compiler was released on or after `2018-12-18`:
+
+```rust
+extern crate version_check as rustc;
+
+match rustc::is_min_date("2018-12-18") {
+    Some(true) => "Yep! It's recent!",
+    Some(false) => "No, it's older.",
+    None => "Couldn't determine the rustc version."
+};
+```
+
+Check that the running compiler supports feature flags:
+
+```rust
+extern crate version_check as rustc;
+
+match rustc::is_feature_flaggable() {
+    Some(true) => "Yes! It's a dev or nightly release!",
+    Some(false) => "No, it's stable or beta.",
+    None => "Couldn't determine the rustc version."
+};
+```
+
+See the [rustdocs](https://docs.rs/version_check) for more examples and complete
+documentation.
+
+## Alternatives
+
+This crate is dead simple with no dependencies. If you need something more and
+don't care about panicking if the version cannot be obtained, or if you don't
+mind adding dependencies, see [rustc_version]. If you'd instead prefer a feature
+detection library that works by dynamically invoking `rustc` with a
+representative code sample, see [autocfg].
+
+[rustc_version]: https://crates.io/crates/rustc_version
+[autocfg]: https://crates.io/crates/autocfg
+
+## License
+
+`version_check` is licensed under either of the following, at your option:
+
+ * 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)