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-graphics-core/README.tpl

@@ -0,0 +1,38 @@
+<p align="center">
+    <img width="150" src="https://raw.githubusercontent.com/embedded-graphics/embedded-graphics/b225511f390c0ed9bc065eb67d05125845312148/assets/logo_core.svg?sanitize=true" alt="Embedded graphics logo">
+</p>
+<p align="center">
+    <a href="https://circleci.com/gh/embedded-graphics/embedded-graphics/tree/master"><img src="https://circleci.com/gh/embedded-graphics/embedded-graphics/tree/master.svg?style=shield" alt="Build Status"></a>
+    <a href="https://crates.io/crates/embedded-graphics-core"><img src="https://img.shields.io/crates/v/embedded-graphics-core.svg" alt="Crates.io"></a>
+    <a href="https://docs.rs/embedded-graphics-core"><img src="https://docs.rs/embedded-graphics-core/badge.svg" alt="Docs.rs"></a>
+    <a href="https://matrix.to/#/#rust-embedded-graphics:matrix.org"><img src="https://img.shields.io/matrix/rust-embedded-graphics:matrix.org" alt="embedded-graphics on Matrix"></a>
+</p>
+
+# Embedded graphics core
+
+{{readme}}
+
+## Minimum supported Rust version
+
+The minimum supported Rust version for embedded-graphics-core is `1.71.1` or greater.
+Ensure you have the correct version of Rust installed, preferably through <https://rustup.rs>.
+
+## Development setup
+
+Please see the [development setup guide](../doc/development-setup.md).
+
+## 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.