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/esp32s3/build.rs

@@ -0,0 +1,17 @@
+#![doc = r" Builder file for Peripheral access crate generated by svd2rust tool"]
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::PathBuf;
+fn main() {
+    if env::var_os("CARGO_FEATURE_RT").is_some() {
+        let out = &PathBuf::from(env::var_os("OUT_DIR").unwrap());
+        File::create(out.join("device.x"))
+            .unwrap()
+            .write_all(include_bytes!("device.x"))
+            .unwrap();
+        println!("cargo:rustc-link-search={}", out.display());
+        println!("cargo:rerun-if-changed=device.x");
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+}