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/heapless/suppressions.txt

@@ -0,0 +1,10 @@
+race:std::panic::catch_unwind
+race:std::thread::scope
+
+# std::thread::spawn false positive; seen on Ubuntu 20.04 but not on Arch Linux (2022-04-29)
+race:drop_in_place*JoinHandle
+race:alloc::sync::Arc<*>::drop_slow
+race:__call_tls_dtors
+
+# false positives in memcpy (?)
+race:*memcpy*