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/thiserror/tests/ui/lifetime.stderr

@@ -0,0 +1,11 @@
+error: non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static
+ --> tests/ui/lifetime.rs:6:26
+  |
+6 | struct Error<'a>(#[from] Inner<'a>);
+  |                          ^^^^^^^^^
+
+error: non-static lifetimes are not allowed in the source of an error, because std::error::Error requires the source is dyn Error + 'static
+  --> tests/ui/lifetime.rs:15:17
+   |
+15 |     Foo(#[from] Generic<&'a str>),
+   |                 ^^^^^^^^^^^^^^^^