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/nrf52840-pac/src/qspi/psel.rs

@@ -0,0 +1,41 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct PSEL {
+    #[doc = "0x00 - Pin select for serial clock SCK"]
+    pub sck: SCK,
+    #[doc = "0x04 - Pin select for chip select signal CSN."]
+    pub csn: CSN,
+    _reserved2: [u8; 0x04],
+    #[doc = "0x0c - Pin select for serial data MOSI/IO0."]
+    pub io0: IO0,
+    #[doc = "0x10 - Pin select for serial data MISO/IO1."]
+    pub io1: IO1,
+    #[doc = "0x14 - Pin select for serial data IO2."]
+    pub io2: IO2,
+    #[doc = "0x18 - Pin select for serial data IO3."]
+    pub io3: IO3,
+}
+#[doc = "SCK (rw) register accessor: an alias for `Reg<SCK_SPEC>`"]
+pub type SCK = crate::Reg<sck::SCK_SPEC>;
+#[doc = "Pin select for serial clock SCK"]
+pub mod sck;
+#[doc = "CSN (rw) register accessor: an alias for `Reg<CSN_SPEC>`"]
+pub type CSN = crate::Reg<csn::CSN_SPEC>;
+#[doc = "Pin select for chip select signal CSN."]
+pub mod csn;
+#[doc = "IO0 (rw) register accessor: an alias for `Reg<IO0_SPEC>`"]
+pub type IO0 = crate::Reg<io0::IO0_SPEC>;
+#[doc = "Pin select for serial data MOSI/IO0."]
+pub mod io0;
+#[doc = "IO1 (rw) register accessor: an alias for `Reg<IO1_SPEC>`"]
+pub type IO1 = crate::Reg<io1::IO1_SPEC>;
+#[doc = "Pin select for serial data MISO/IO1."]
+pub mod io1;
+#[doc = "IO2 (rw) register accessor: an alias for `Reg<IO2_SPEC>`"]
+pub type IO2 = crate::Reg<io2::IO2_SPEC>;
+#[doc = "Pin select for serial data IO2."]
+pub mod io2;
+#[doc = "IO3 (rw) register accessor: an alias for `Reg<IO3_SPEC>`"]
+pub type IO3 = crate::Reg<io3::IO3_SPEC>;
+#[doc = "Pin select for serial data IO3."]
+pub mod io3;