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/ident_case/README.md

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+[![Build Status](https://travis-ci.org/TedDriggs/ident_case.svg?branch=master)](https://travis-ci.org/TedDriggs/ident_case)
+
+Crate for manipulating case of identifiers in Rust programs.
+
+# Features
+* Supports `snake_case`, `lowercase`, `camelCase`, 
+  `PascalCase`, `SCREAMING_SNAKE_CASE`, and `kebab-case`
+* Rename variants, and fields
+
+# Examples
+```rust
+assert_eq!("helloWorld", RenameRule::CamelCase.apply_to_field("hello_world"));
+
+assert_eq!("i_love_serde", RenameRule::SnakeCase.apply_to_variant("ILoveSerde"));
+```