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/nrf51-pac/src/ficr/codepagesize.rs

@@ -0,0 +1,31 @@
+#[doc = "Register `CODEPAGESIZE` reader"]
+pub struct R(crate::R<CODEPAGESIZE_SPEC>);
+impl core::ops::Deref for R {
+    type Target = crate::R<CODEPAGESIZE_SPEC>;
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+impl From<crate::R<CODEPAGESIZE_SPEC>> for R {
+    #[inline(always)]
+    fn from(reader: crate::R<CODEPAGESIZE_SPEC>) -> Self {
+        R(reader)
+    }
+}
+#[doc = "Code memory page size in bytes.\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 [codepagesize](index.html) module"]
+pub struct CODEPAGESIZE_SPEC;
+impl crate::RegisterSpec for CODEPAGESIZE_SPEC {
+    type Ux = u32;
+}
+#[doc = "`read()` method returns [codepagesize::R](R) reader structure"]
+impl crate::Readable for CODEPAGESIZE_SPEC {
+    type Reader = R;
+}
+#[doc = "`reset()` method sets CODEPAGESIZE to value 0xffff_ffff"]
+impl crate::Resettable for CODEPAGESIZE_SPEC {
+    #[inline(always)]
+    fn reset_value() -> Self::Ux {
+        0xffff_ffff
+    }
+}