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/anyhow/tests/ui/must-use.stderr

@@ -0,0 +1,15 @@
+error: unused return value of `anyhow::__private::must_use` that must be used
+ --> tests/ui/must-use.rs:8:9
+  |
+8 |         anyhow!("it failed");
+  |         ^^^^^^^^^^^^^^^^^^^^
+  |
+note: the lint level is defined here
+ --> tests/ui/must-use.rs:1:9
+  |
+1 | #![deny(unused_must_use)]
+  |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+  |
+8 |         let _ = anyhow!("it failed");
+  |         +++++++