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/esp-backtrace/build.rs

@@ -0,0 +1,15 @@
+use esp_build::assert_unique_used_features;
+
+fn main() {
+    // Ensure that only a single chip is specified:
+    assert_unique_used_features!(
+        "esp32", "esp32c2", "esp32c3", "esp32c6", "esp32h2", "esp32p4", "esp32s2", "esp32s3"
+    );
+
+    // Ensure that exactly a backend is selected:
+    assert_unique_used_features!("defmt", "println");
+
+    if cfg!(feature = "custom-halt") && cfg!(feature = "halt-cores") {
+        panic!("Only one of `custom-halt` and `halt-cores` can be enabled");
+    }
+}