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>

Justfile

@@ -1,8 +1,8 @@
 set shell := ["bash", "-c"]
 
-# Board definitions: feature target chip [extra-cargo-args]
-# Boards requiring the esp toolchain are marked — check-all skips them gracefully
-# if the toolchain isn't installed.
+# Board definitions: feature target chip
+# Xtensa targets require the esp toolchain (espup); check-all gracefully skips
+# boards whose toolchain isn't installed.
 heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
 rak_4631  := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"