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/regulators_ns/mainregstatus.rs

@@ -0,0 +1,74 @@
+#[doc = "Register `MAINREGSTATUS` reader"]
+pub struct R(crate::R<MAINREGSTATUS_SPEC>);
+impl core::ops::Deref for R {
+    type Target = crate::R<MAINREGSTATUS_SPEC>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+impl From<crate::R<MAINREGSTATUS_SPEC>> for R {
+    #[inline(always)]
+    fn from(reader: crate::R<MAINREGSTATUS_SPEC>) -> Self {
+        R(reader)
+    }
+}
+#[doc = "Field `VREGH` reader - VREGH status"]
+pub type VREGH_R = crate::BitReader<VREGH_A>;
+#[doc = "VREGH status\n\nValue on reset: 0"]
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum VREGH_A {
+    #[doc = "0: Normal voltage mode. Voltage supplied on VDD and VDDH."]
+    INACTIVE = 0,
+    #[doc = "1: High voltage mode. Voltage supplied on VDDH."]
+    ACTIVE = 1,
+}
+impl From<VREGH_A> for bool {
+    #[inline(always)]
+    fn from(variant: VREGH_A) -> Self {
+        variant as u8 != 0
+    }
+}
+impl VREGH_R {
+    #[doc = "Get enumerated values variant"]
+    #[inline(always)]
+    pub fn variant(&self) -> VREGH_A {
+        match self.bits {
+            false => VREGH_A::INACTIVE,
+            true => VREGH_A::ACTIVE,
+        }
+    }
+    #[doc = "Checks if the value of the field is `INACTIVE`"]
+    #[inline(always)]
+    pub fn is_inactive(&self) -> bool {
+        *self == VREGH_A::INACTIVE
+    }
+    #[doc = "Checks if the value of the field is `ACTIVE`"]
+    #[inline(always)]
+    pub fn is_active(&self) -> bool {
+        *self == VREGH_A::ACTIVE
+    }
+}
+impl R {
+    #[doc = "Bit 0 - VREGH status"]
+    #[inline(always)]
+    pub fn vregh(&self) -> VREGH_R {
+        VREGH_R::new((self.bits & 1) != 0)
+    }
+}
+#[doc = "Main supply status\n\nThis register you can [`read`](crate::generic::Reg::read). See [API](https://docs.rs/svd2rust/#read--modify--write-api).\n\nFor information about available fields see [mainregstatus](index.html) module"]
+pub struct MAINREGSTATUS_SPEC;
+impl crate::RegisterSpec for MAINREGSTATUS_SPEC {
+    type Ux = u32;
+}
+#[doc = "`read()` method returns [mainregstatus::R](R) reader structure"]
+impl crate::Readable for MAINREGSTATUS_SPEC {
+    type Reader = R;
+}
+#[doc = "`reset()` method sets MAINREGSTATUS to value 0"]
+impl crate::Resettable for MAINREGSTATUS_SPEC {
+    #[inline(always)]
+    fn reset_value() -> Self::Ux {
+        0
+    }
+}