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/ssd1306/.cargo/config.toml

@@ -0,0 +1,10 @@
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+runner = [ "probe-rs", "run", "--chip", "STM32F103C8" ]
+rustflags = [
+  "-C", "link-arg=-Tlink.x",
+  "-C", "link-arg=--nmagic",
+  "-C", "link-arg=-Tdefmt.x",
+]
+
+[build]
+target = "thumbv7m-none-eabi"