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-macro-error/build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    if !version_check::is_feature_flaggable().unwrap_or(false) {
+        println!("cargo:rustc-cfg=use_fallback");
+    }
+
+    if version_check::is_max_version("1.38.0").unwrap_or(false)
+        || !version_check::Channel::read().unwrap().is_stable()
+    {
+        println!("cargo:rustc-cfg=skip_ui_tests");
+    }
+}