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/darling/tests/compile-fail/from_expr.stderr

@@ -0,0 +1,11 @@
+error: `from_expr` cannot be used on enums with non-skipped unit variants because it conflicts with the generated impl
+  --> tests/compile-fail/from_expr.rs:16:23
+   |
+16 | #[darling(from_expr = |expr| Ok(HasUnits::Variant2))]
+   |                       ^
+
+error: `from_expr` cannot be used on newtype structs because the implementation is entirely delegated to the inner type
+  --> tests/compile-fail/from_expr.rs:29:23
+   |
+29 | #[darling(from_expr = newtype_from_expr)]
+   |                       ^^^^^^^^^^^^^^^^^