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/ctrlap_ns/mailbox.rs

@@ -0,0 +1,29 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct MAILBOX {
+    #[doc = "0x00 - Data sent from the debugger to the CPU."]
+    pub rxdata: RXDATA,
+    #[doc = "0x04 - This register shows a status that indicates if data sent from the debugger to the CPU has been read."]
+    pub rxstatus: RXSTATUS,
+    _reserved2: [u8; 0x78],
+    #[doc = "0x80 - Data sent from the CPU to the debugger."]
+    pub txdata: TXDATA,
+    #[doc = "0x84 - This register shows a status that indicates if the data sent from the CPU to the debugger has been read."]
+    pub txstatus: TXSTATUS,
+}
+#[doc = "RXDATA (r) register accessor: an alias for `Reg<RXDATA_SPEC>`"]
+pub type RXDATA = crate::Reg<rxdata::RXDATA_SPEC>;
+#[doc = "Data sent from the debugger to the CPU."]
+pub mod rxdata;
+#[doc = "RXSTATUS (r) register accessor: an alias for `Reg<RXSTATUS_SPEC>`"]
+pub type RXSTATUS = crate::Reg<rxstatus::RXSTATUS_SPEC>;
+#[doc = "This register shows a status that indicates if data sent from the debugger to the CPU has been read."]
+pub mod rxstatus;
+#[doc = "TXDATA (rw) register accessor: an alias for `Reg<TXDATA_SPEC>`"]
+pub type TXDATA = crate::Reg<txdata::TXDATA_SPEC>;
+#[doc = "Data sent from the CPU to the debugger."]
+pub mod txdata;
+#[doc = "TXSTATUS (r) register accessor: an alias for `Reg<TXSTATUS_SPEC>`"]
+pub type TXSTATUS = crate::Reg<txstatus::TXSTATUS_SPEC>;
+#[doc = "This register shows a status that indicates if the data sent from the CPU to the debugger has been read."]
+pub mod txstatus;