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/encode_unicode/README.md

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+# encode_unicode
+
+UTF-8 and UTF-16 character types, iterators and related methods for `char`, `u8` and `u16`.
+
+[![crates.io page](https://img.shields.io/crates/v/encode_unicode.svg)](https://crates.io/crates/encode_unicode/)
+
+## Features
+
+* **[`Utf8Char`](https://docs.rs/encode_unicode/latest/encode_unicode/struct.Utf8Char.html)**:
+  A `char` stored as UTF-8. Can be borrowed as a `str` or `u8` slice.
+* **[`Utf16Char`](https://docs.rs/encode_unicode/latest/encode_unicode/struct.Utf16Char.html)**:
+  A `char` stored as UTF-16. Can be borrowed as an `u16` slice.
+* [Conversion methods on `char`](https://docs.rs/encode_unicode/latest/encode_unicode/trait.CharExt.html):
+  * to and from UTF-8 as `[u8; 4]` or slice.
+  * to and from UTF-16 as `(u16, Option<u16>)` or slice.
+* [Iterator adapters](https://docs.rs/encode_unicode/latest/encode_unicode/trait.IterExt.html)
+  for converting betwenn `u8`s and `Utf8Char`s or `u16`s and `Utf16Char`s.
+* Optimized [slice-based decoding iterators](https://docs.rs/encode_unicode/latest/encode_unicode/trait.SliceExt.html).
+* [Precise errors when decoding a char from UTF-8, UTF-16 or `u32` fails](http://docs.rs/encode_unicode/latest/encode_unicode/error/index.html).
+* Utility methods on [`u8`](https://docs.rs/encode_unicode/latest/encode_unicode/trait.U8UtfExt.html)
+  and [`u16`](https://docs.rs/encode_unicode/latest/encode_unicode/trait.U16UtfExt.html).
+
+The minimum supported version of Rust is 1.15,
+older versions might work now but can break with a minor update.
+
+## Optional features
+
+* `#![no_std]`-mode: There are a few differences:
+  * `Error` doesn't exist, but `description()` is made available as an inherent impl.
+  * `Extend`/`FromIterator`-implementations for `String`/`Vec<u8>`/`Vec<u16>` are missing.
+  * There is no `io`, so `Utf8Iterator` and `Utf8CharSplitter` doesn't implement `Read`.
+  This feature is enabled by setting `default-features=false` in `Cargo.toml`:
+  `encode_unicode = {version="0.3.4", default-features=false}`.
+* Integration with the [ascii](https://tomprogrammer.github.io/rust-ascii/ascii/index.html) crate:
+  Convert `Utf8Char` and `Utf16Char` to and from [ascii::`AsciiChar`](https://tomprogrammer.github.io/rust-ascii/ascii/enum.AsciiChar.html).
+
+## 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.
+
+## History
+
+The original purpose of this crate was to provide standins for the then
+unstable `encode_utf8()` and `encode_utf16()`.  
+The standins were removed in 0.3 when Rust 1.15 stabilized the `encode_`
+methods, but the other stuff I added, such as iterators like
+those `encode_utf{8,16}() returned for a while, might still be of use.