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/nrf52820-pac/src/approtect.rs

@@ -0,0 +1,18 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0550],
+    #[doc = "0x550 - Software force enable APPROTECT mechanism until next reset."]
+    pub forceprotect: FORCEPROTECT,
+    _reserved1: [u8; 0x04],
+    #[doc = "0x558 - Software disable APPROTECT mechanism"]
+    pub disable: DISABLE,
+}
+#[doc = "FORCEPROTECT (rw) register accessor: an alias for `Reg<FORCEPROTECT_SPEC>`"]
+pub type FORCEPROTECT = crate::Reg<forceprotect::FORCEPROTECT_SPEC>;
+#[doc = "Software force enable APPROTECT mechanism until next reset."]
+pub mod forceprotect;
+#[doc = "DISABLE (rw) register accessor: an alias for `Reg<DISABLE_SPEC>`"]
+pub type DISABLE = crate::Reg<disable::DISABLE_SPEC>;
+#[doc = "Software disable APPROTECT mechanism"]
+pub mod disable;