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/minijinja/src/compiler/mod.rs

@@ -0,0 +1,9 @@
+#![allow(missing_docs)]
+/// This module contains the internals of the compiler.
+pub mod ast;
+pub mod codegen;
+pub mod instructions;
+pub mod lexer;
+pub mod meta;
+pub mod parser;
+pub mod tokens;