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/proc-macro-error/tests/ui/option_ext.stderr

@@ -0,0 +1,7 @@
+error: Option::expect_or_abort() test
+ --> $DIR/option_ext.rs:4:1
+  |
+4 | option_ext!(one, two);
+  | ^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)