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/uarte0_ns/psel.rs

@@ -0,0 +1,28 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct PSEL {
+    #[doc = "0x00 - Pin select for RTS signal"]
+    pub rts: RTS,
+    #[doc = "0x04 - Pin select for TXD signal"]
+    pub txd: TXD,
+    #[doc = "0x08 - Pin select for CTS signal"]
+    pub cts: CTS,
+    #[doc = "0x0c - Pin select for RXD signal"]
+    pub rxd: RXD,
+}
+#[doc = "RTS (rw) register accessor: an alias for `Reg<RTS_SPEC>`"]
+pub type RTS = crate::Reg<rts::RTS_SPEC>;
+#[doc = "Pin select for RTS signal"]
+pub mod rts;
+#[doc = "TXD (rw) register accessor: an alias for `Reg<TXD_SPEC>`"]
+pub type TXD = crate::Reg<txd::TXD_SPEC>;
+#[doc = "Pin select for TXD signal"]
+pub mod txd;
+#[doc = "CTS (rw) register accessor: an alias for `Reg<CTS_SPEC>`"]
+pub type CTS = crate::Reg<cts::CTS_SPEC>;
+#[doc = "Pin select for CTS signal"]
+pub mod cts;
+#[doc = "RXD (rw) register accessor: an alias for `Reg<RXD_SPEC>`"]
+pub type RXD = crate::Reg<rxd::RXD_SPEC>;
+#[doc = "Pin select for RXD signal"]
+pub mod rxd;