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-time/src/lib.rs

@@ -0,0 +1,65 @@
+#![cfg_attr(not(any(feature = "std", feature = "wasm", test)), no_std)]
+#![allow(async_fn_in_trait)]
+#![doc = include_str!("../README.md")]
+#![allow(clippy::new_without_default)]
+#![warn(missing_docs)]
+
+//! ## Feature flags
+#![doc = document_features::document_features!(feature_label = r#"<span class="stab portability"><code>{feature}</code></span>"#)]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+mod delay;
+mod duration;
+mod instant;
+mod timer;
+
+#[cfg(feature = "mock-driver")]
+mod driver_mock;
+
+#[cfg(feature = "mock-driver")]
+pub use driver_mock::MockDriver;
+
+#[cfg(feature = "std")]
+mod driver_std;
+#[cfg(feature = "wasm")]
+mod driver_wasm;
+#[cfg(feature = "generic-queue")]
+mod queue_generic;
+
+pub use delay::{block_for, Delay};
+pub use duration::Duration;
+pub use embassy_time_driver::TICK_HZ;
+pub use instant::Instant;
+pub use timer::{with_deadline, with_timeout, Ticker, TimeoutError, Timer, WithTimeout};
+
+const fn gcd(a: u64, b: u64) -> u64 {
+    if b == 0 {
+        a
+    } else {
+        gcd(b, a % b)
+    }
+}
+
+pub(crate) const GCD_1K: u64 = gcd(TICK_HZ, 1_000);
+pub(crate) const GCD_1M: u64 = gcd(TICK_HZ, 1_000_000);
+pub(crate) const GCD_1G: u64 = gcd(TICK_HZ, 1_000_000_000);
+
+#[cfg(feature = "defmt-timestamp-uptime-s")]
+defmt::timestamp! {"{=u64}", Instant::now().as_secs() }
+
+#[cfg(feature = "defmt-timestamp-uptime-ms")]
+defmt::timestamp! {"{=u64:ms}", Instant::now().as_millis() }
+
+#[cfg(any(feature = "defmt-timestamp-uptime", feature = "defmt-timestamp-uptime-us"))]
+defmt::timestamp! {"{=u64:us}", Instant::now().as_micros() }
+
+#[cfg(feature = "defmt-timestamp-uptime-ts")]
+defmt::timestamp! {"{=u64:ts}", Instant::now().as_secs() }
+
+#[cfg(feature = "defmt-timestamp-uptime-tms")]
+defmt::timestamp! {"{=u64:tms}", Instant::now().as_millis() }
+
+#[cfg(feature = "defmt-timestamp-uptime-tus")]
+defmt::timestamp! {"{=u64:tus}", Instant::now().as_micros() }