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/nrf5340-net-pac/src/vreqctrl_ns.rs

@@ -0,0 +1,12 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0500],
+    #[doc = "0x500..0x50c - Unspecified"]
+    pub vregradio: VREGRADIO,
+}
+#[doc = "Unspecified"]
+pub use vregradio::VREGRADIO;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod vregradio;