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/nrf9160-pac/src/regulators_ns.rs

@@ -0,0 +1,18 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0500],
+    #[doc = "0x500 - System OFF register"]
+    pub systemoff: SYSTEMOFF,
+    _reserved1: [u8; 0x74],
+    #[doc = "0x578 - Enable DC/DC mode of the main voltage regulator."]
+    pub dcdcen: DCDCEN,
+}
+#[doc = "SYSTEMOFF (w) register accessor: an alias for `Reg<SYSTEMOFF_SPEC>`"]
+pub type SYSTEMOFF = crate::Reg<systemoff::SYSTEMOFF_SPEC>;
+#[doc = "System OFF register"]
+pub mod systemoff;
+#[doc = "DCDCEN (rw) register accessor: an alias for `Reg<DCDCEN_SPEC>`"]
+pub type DCDCEN = crate::Reg<dcdcen::DCDCEN_SPEC>;
+#[doc = "Enable DC/DC mode of the main voltage regulator."]
+pub mod dcdcen;