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/oscillators_ns.rs

@@ -0,0 +1,19 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x05c4],
+    #[doc = "0x5c4 - Programmable capacitance of XC1 and XC2"]
+    pub xosc32mcaps: XOSC32MCAPS,
+    _reserved1: [u8; 0xf8],
+    #[doc = "0x6c0..0x6d4 - Unspecified"]
+    pub xosc32ki: XOSC32KI,
+}
+#[doc = "XOSC32MCAPS (rw) register accessor: an alias for `Reg<XOSC32MCAPS_SPEC>`"]
+pub type XOSC32MCAPS = crate::Reg<xosc32mcaps::XOSC32MCAPS_SPEC>;
+#[doc = "Programmable capacitance of XC1 and XC2"]
+pub mod xosc32mcaps;
+#[doc = "Unspecified"]
+pub use xosc32ki::XOSC32KI;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod xosc32ki;