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/embedded-io/README.md

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+[![crates.io](https://img.shields.io/crates/d/embedded-io.svg)](https://crates.io/crates/embedded-io)
+[![crates.io](https://img.shields.io/crates/v/embedded-io.svg)](https://crates.io/crates/embedded-io)
+[![Documentation](https://docs.rs/embedded-io/badge.svg)](https://docs.rs/embedded-io)
+
+# `embedded-io`
+
+This project is developed and maintained by the [HAL team](https://github.com/rust-embedded/wg#the-hal-team).
+
+Input/Output traits for embedded systems.
+
+Rust's `std::io` traits are not available in `no_std` targets, mainly because `std::io::Error`
+requires allocation. This crate contains replacement equivalent traits, usable in `no_std`
+targets.
+
+## Differences with `std::io`
+
+- `Error` is an associated type. This allows each implementor to return its own error type,
+while avoiding `dyn` or `Box`. This is consistent with how errors are handled in [`embedded-hal`](https://github.com/rust-embedded/embedded-hal/).
+- In `std::io`, the `Read`/`Write` traits might be blocking or non-blocking (i.e. returning `WouldBlock` errors) depending on the file descriptor's mode, which is only known at run-time. This allows passing a non-blocking stream to code that expects a blocking
+stream, causing unexpected errors. To solve this, `embedded-io` specifies `Read`/`Write` are always blocking, and adds new `ReadReady`/`WriteReady` traits to allow using streams in a non-blocking way.
+
+## Optional Cargo features
+
+- **`std`**: Adds `From` impls to convert to/from `std::io` structs, adds `std::error::Error` impls.
+- **`alloc`**: Adds blanket impls for `Box`, adds `Write` impl to `Vec`.
+- **`defmt-03`**: Derive `defmt::Format` from `defmt` 0.3 for enums and structs.
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.60 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
+See [here](../docs/msrv.md) for details on how the MSRV may be upgraded.
+
+## License
+
+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.