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/toml_edit/README.md

@@ -0,0 +1,59 @@
+# toml_edit
+
+[![Documentation](https://img.shields.io/badge/docs-master-blue.svg)](https://docs.rs/toml_edit)
+![License](https://img.shields.io/crates/l/toml_edit.svg)
+[![Crates Status](https://img.shields.io/crates/v/toml_edit.svg)](https://crates.io/crates/toml_edit)
+
+
+This crate allows you to parse and modify toml
+documents, while preserving comments, spaces *and
+relative order* of items.
+
+`toml_edit` is primarily tailored for [cargo-edit](https://github.com/killercup/cargo-edit/) needs.
+
+## Example
+
+```rust
+use toml_edit::{DocumentMut, value};
+
+fn main() {
+    let toml = r#"
+"hello" = 'toml!' # comment
+['a'.b]
+    "#;
+    let mut doc = toml.parse::<DocumentMut>().expect("invalid doc");
+    assert_eq!(doc.to_string(), toml);
+    // let's add a new key/value pair inside a.b: c = {d = "hello"}
+    doc["a"]["b"]["c"]["d"] = value("hello");
+    // autoformat inline table a.b.c: { d = "hello" }
+    doc["a"]["b"]["c"].as_inline_table_mut().map(|t| t.fmt());
+    let expected = r#"
+"hello" = 'toml!' # comment
+['a'.b]
+c = { d = "hello" }
+    "#;
+    assert_eq!(doc.to_string(), expected);
+}
+```
+
+## Limitations
+
+Things it does not preserve:
+
+* Order of dotted keys, see [issue](https://github.com/toml-rs/toml/issues/163).
+
+## License
+
+Licensed under either of
+
+* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or <https://www.apache.org/licenses/LICENSE-2.0>)
+* MIT license ([LICENSE-MIT](LICENSE-MIT) or <https://opensource.org/license/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.