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/doc/development-setup.md

@@ -0,0 +1,40 @@
+# Development setup
+
+## Ubuntu/Linux Mint
+
+```bash
+# Update to latest stable version of Rust
+rustup update
+
+# Ensure rustfmt is installed
+rustup component add rustfmt
+
+# Install `cargo-readme`, `cargo-deadlinks` and `lychee`
+cargo install cargo-readme cargo-deadlinks lychee
+
+# Install `just`
+cargo install just
+
+# Install SDL2 for simulator for build script
+sudo apt install libsdl2-dev
+
+```
+
+## Generating readmes
+
+The various `README.md` files in this project are generated from each crate's `lib.rs` comment. To
+regenerate a readme, ensure [`cargo-readme`](https://crates.io/crates/cargo-readme) and
+[`just`](https://github.com/casey/just) are installed then run:
+
+```bash
+just generate-readme <crate>
+
+# e.g.
+just generate-readme simulator
+```
+
+All readmes can be generated at the same time by running `just generate-readmes` in the project
+root.
+
+Running `just build` will check if the readme was successfully updated. The updated `README.md`
+should be committed into git.