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/static_cell/README.md

@@ -0,0 +1,63 @@
+# static-cell
+[![crates.io](https://img.shields.io/crates/d/static_cell.svg)](https://crates.io/crates/static_cell)
+[![crates.io](https://img.shields.io/crates/v/static_cell.svg)](https://crates.io/crates/static_cell)
+[![Documentation](https://docs.rs/static_cell/badge.svg)](https://docs.rs/static_cell)
+
+Statically allocated, initialized at runtime cell.
+
+`StaticCell` provides a no-std-compatible, no-alloc way to reserve memory at compile time for
+a value, but initialize it at runtime, and get a `'static` reference to it.
+
+This is useful in the following scenarios:
+
+- You need `&'static T`, but `T` can't be constructed in const context so you can't simply use a `static`.
+- You need `&'static mut T`, not just `&'static T`.
+
+## Example
+
+```rust
+use static_cell::StaticCell;
+
+// Statically allocate memory for a `u32`.
+static SOME_INT: StaticCell<u32> = StaticCell::new();
+
+// Initialize it at runtime. This returns a `&'static mut`.
+let x: &'static mut u32 = SOME_INT.init(42);
+assert_eq!(*x, 42);
+
+// Trying to call `.init()` again would panic, because the StaticCell is already initialized.
+// SOME_INT.init(42);
+```
+
+## Alternatives
+
+- If you can use `alloc`, you can use `Box::leak()`.
+- If you're OK with `unsafe`, you can use `static mut THING: MaybeUninit<T>`.
+- If you need just `&'static T` (instead of `&'static mut T`), there's [`OnceCell`](https://doc.rust-lang.org/stable/core/cell/struct.OnceCell.html) (not thread-safe though) or [`OnceLock`](https://doc.rust-lang.org/stable/std/sync/struct.OnceLock.html) (thread-safe, but requires `std`).
+
+## Interoperability
+
+This crate uses [`portable-atomic`](https://crates.io/crates/portable-atomic), so on targets without native
+atomics you must import a crate that provides a [`critical-section`](https://github.com/rust-embedded/critical-section) 
+implementation. See the `critical-section` README for details.
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.56 and up. It might compile with
+older versions but that may change in any new patch release.
+
+## License
+
+This work is 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.