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>

Cargo.toml

@@ -19,7 +19,14 @@ postcard = { version = "1.0", default-features = false }
 serde = { version = "1.0", default-features = false, features = ["derive"] }
 static_cell = "2.1"
 
-# ESP32-S3 (Heltec V3) — builds with nightly + -Zbuild-std=core
+# Display rendering
+embedded-graphics = "0.8"
+ssd1306 = { version = "0.10", features = ["async"] }
+
+# Async combinators (select)
+embassy-futures = "0.1"
+
+# ESP32-S3 (Heltec V3) — requires esp toolchain (espup)
 esp-backtrace = { version = "0.14", optional = true, features = ["esp32s3", "panic-handler", "defmt"] }
 esp-hal = { version = "0.22", optional = true, features = ["esp32s3"] }
 esp-hal-embassy = { version = "0.5", optional = true, features = ["esp32s3"] }