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.rs

@@ -0,0 +1,49 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0428],
+    #[doc = "0x428 - Main supply status"]
+    pub mainregstatus: MAINREGSTATUS,
+    _reserved1: [u8; 0xd4],
+    #[doc = "0x500 - System OFF register"]
+    pub systemoff: SYSTEMOFF,
+    _reserved2: [u8; 0x0c],
+    #[doc = "0x510 - Power-fail comparator configuration"]
+    pub pofcon: POFCON,
+    _reserved3: [u8; 0x01f0],
+    #[doc = "0x704 - Unspecified"]
+    pub vregmain: VREGMAIN,
+    _reserved4: [u8; 0x01f8],
+    #[doc = "0x900..0x908 - Unspecified"]
+    pub vregradio: VREGRADIO,
+    _reserved5: [u8; 0x01f8],
+    #[doc = "0xb00 - Unspecified"]
+    pub vregh: VREGH,
+}
+#[doc = "MAINREGSTATUS (r) register accessor: an alias for `Reg<MAINREGSTATUS_SPEC>`"]
+pub type MAINREGSTATUS = crate::Reg<mainregstatus::MAINREGSTATUS_SPEC>;
+#[doc = "Main supply status"]
+pub mod mainregstatus;
+#[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 = "POFCON (rw) register accessor: an alias for `Reg<POFCON_SPEC>`"]
+pub type POFCON = crate::Reg<pofcon::POFCON_SPEC>;
+#[doc = "Power-fail comparator configuration"]
+pub mod pofcon;
+#[doc = "Unspecified"]
+pub use vregmain::VREGMAIN;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod vregmain;
+#[doc = "Unspecified"]
+pub use vregradio::VREGRADIO;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod vregradio;
+#[doc = "Unspecified"]
+pub use vregh::VREGH;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod vregh;