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/bitflags/CONTRIBUTING.md

@@ -0,0 +1,9 @@
+# Updating compile-fail test outputs
+
+`bitflags` uses the `trybuild` crate to integration test its macros. Since Rust error messages change frequently enough that `nightly` builds produce spurious failures, we only check the compiler output in `beta` builds. If you run:
+
+```
+TRYBUILD=overwrite cargo +beta test --all
+```
+
+it will run the tests and update the `trybuild` output files.