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/litrs/README.md

@@ -0,0 +1,85 @@
+# `litrs`: parsing and inspecting Rust literals
+
+[<img alt="CI status of main" src="https://img.shields.io/github/actions/workflow/status/LukasKalbertodt/litrs/ci.yml?branch=main&label=CI&logo=github&logoColor=white&style=for-the-badge" height="23">](https://github.com/LukasKalbertodt/litrs/actions/workflows/ci.yml)
+[<img alt="Crates.io Version" src="https://img.shields.io/crates/v/litrs?logo=rust&style=for-the-badge" height="23">](https://crates.io/crates/litrs)
+[<img alt="docs.rs" src="https://img.shields.io/crates/v/litrs?color=blue&label=docs&style=for-the-badge" height="23">](https://docs.rs/litrs)
+
+`litrs` offers functionality to parse Rust literals, i.e. tokens in the Rust programming language that represent fixed values.
+For example: `27`, `"crab"`, `bool`.
+This is particularly useful for proc macros, but can also be used outside of a proc-macro context.
+
+**Why this library?**
+Unfortunately, the `proc_macro` API shipped with the compiler offers no easy way to inspect literals.
+There are mainly two libraries for this purpose:
+[`syn`](https://github.com/dtolnay/syn) and [`literalext`](https://github.com/mystor/literalext).
+The latter is deprecated.
+And `syn` is oftentimes overkill for the task at hand, especially when developing function-like proc-macros (e.g. `foo!(..)`).
+This crate is a lightweight alternative.
+Also, when it comes to literals, `litrs` offers a bit more flexibility and a few more features compared to `syn`.
+
+## Example
+
+### In proc macro
+
+```rust
+use std::convert::TryFrom;
+use proc_macro::TokenStream;
+use litrs::Literal;
+
+#[proc_macro]
+pub fn foo(input: TokenStream) -> TokenStream {
+    // Please do proper error handling in your real code!
+    let first_token = input.into_iter().next().expect("no input");
+
+    // `try_from` will return an error if the token is not a literal.
+    match Literal::try_from(first_token) {
+        // Convenient methods to produce decent errors via `compile_error!`.
+        Err(e) => return e.to_compile_error(),
+
+        // You can now inspect your literal!
+        Ok(Literal::Integer(i)) => {
+            println!("Got an integer specified in base {:?}", i.base());
+
+            let value = i.value::<u64>().expect("integer literal too large");
+            println!("Is your integer even? {}", value % 2 == 0);
+        }
+        Ok(other) => {
+            println!("Got a non-integer literal");
+        }
+    }
+
+    TokenStream::new() // dummy output
+}
+```
+
+If you are expecting a specific kind of literal, you can also use this, which will return an error if the token is not a float literal.
+
+```rust
+FloatLit::try_from(first_token)
+```
+
+### Parsing from a `&str`
+
+Outside of a proc macro context you might want to parse a string directly.
+
+```rust
+use litrs::{FloatLit, Literal};
+
+let lit = Literal::parse("'🦀'").expect("failed to parse literal");
+let float_lit = FloatLit::parse("2.7e3").expect("failed to parse as float literal");
+```
+
+See [**the documentation**](https://docs.rs/litrs) or the `examples/` directory for more examples and information.
+
+
+<br />
+
+---
+
+## License
+
+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.
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this project by you, as defined in the Apache-2.0 license,
+shall be dual licensed as above, without any additional terms or conditions.