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/proc-macro2/src/num.rs

@@ -0,0 +1,17 @@
+// TODO: use NonZero<char> in Rust 1.89+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct NonZeroChar(char);
+
+impl NonZeroChar {
+    pub fn new(ch: char) -> Option<Self> {
+        if ch == '\0' {
+            None
+        } else {
+            Some(NonZeroChar(ch))
+        }
+    }
+
+    pub fn get(self) -> char {
+        self.0
+    }
+}