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/rustversion/src/time.rs

@@ -0,0 +1,51 @@
+use crate::date::Date;
+use std::env;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+// Timestamp of 2016-03-01 00:00:00 in UTC.
+const BASE: u64 = 1456790400;
+const BASE_YEAR: u16 = 2016;
+const BASE_MONTH: u8 = 3;
+
+// Days between leap days.
+const CYCLE: u64 = 365 * 4 + 1;
+
+const DAYS_BY_MONTH: [u8; 12] = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+pub fn today() -> Date {
+    let default = Date {
+        year: 2025,
+        month: 2,
+        day: 25,
+    };
+    try_today().unwrap_or(default)
+}
+
+fn try_today() -> Option<Date> {
+    if let Some(pkg_name) = env::var_os("CARGO_PKG_NAME") {
+        if pkg_name.to_str() == Some("rustversion-tests") {
+            return None; // Stable date for ui testing.
+        }
+    }
+
+    let now = SystemTime::now();
+    let since_epoch = now.duration_since(UNIX_EPOCH).ok()?;
+    let secs = since_epoch.as_secs();
+
+    let approx_days = secs.checked_sub(BASE)? / 60 / 60 / 24;
+    let cycle = approx_days / CYCLE;
+    let mut rem = approx_days % CYCLE;
+
+    let mut year = BASE_YEAR + cycle as u16 * 4;
+    let mut month = BASE_MONTH;
+    loop {
+        let days_in_month = DAYS_BY_MONTH[month as usize - 1];
+        if rem < days_in_month as u64 {
+            let day = rem as u8 + 1;
+            return Some(Date { year, month, day });
+        }
+        rem -= days_in_month as u64;
+        year += (month == 12) as u16;
+        month = month % 12 + 1;
+    }
+}