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-app-pac/src/saadc_ns/events_ch.rs

@@ -0,0 +1,16 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct EVENTS_CH {
+    #[doc = "0x00 - Description cluster: Last results is equal or above CH\\[n\\].LIMIT.HIGH"]
+    pub limith: LIMITH,
+    #[doc = "0x04 - Description cluster: Last results is equal or below CH\\[n\\].LIMIT.LOW"]
+    pub limitl: LIMITL,
+}
+#[doc = "LIMITH (rw) register accessor: an alias for `Reg<LIMITH_SPEC>`"]
+pub type LIMITH = crate::Reg<limith::LIMITH_SPEC>;
+#[doc = "Description cluster: Last results is equal or above CH\\[n\\].LIMIT.HIGH"]
+pub mod limith;
+#[doc = "LIMITL (rw) register accessor: an alias for `Reg<LIMITL_SPEC>`"]
+pub type LIMITL = crate::Reg<limitl::LIMITL_SPEC>;
+#[doc = "Description cluster: Last results is equal or below CH\\[n\\].LIMIT.LOW"]
+pub mod limitl;