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/embassy-sync/src/lib.rs

@@ -0,0 +1,26 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+#![allow(async_fn_in_trait)]
+#![allow(clippy::new_without_default)]
+#![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+// internal use
+mod ring_buffer;
+
+pub mod blocking_mutex;
+pub mod channel;
+pub mod lazy_lock;
+pub mod mutex;
+pub mod once_lock;
+pub mod pipe;
+pub mod priority_channel;
+pub mod pubsub;
+pub mod rwlock;
+pub mod semaphore;
+pub mod signal;
+pub mod waitqueue;
+pub mod watch;
+pub mod zerocopy_channel;