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/nrf52832-pac/src/spi0/psel.rs

@@ -0,0 +1,22 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct PSEL {
+    #[doc = "0x00 - Pin select for SCK"]
+    pub sck: SCK,
+    #[doc = "0x04 - Pin select for MOSI"]
+    pub mosi: MOSI,
+    #[doc = "0x08 - Pin select for MISO"]
+    pub miso: MISO,
+}
+#[doc = "SCK (rw) register accessor: an alias for `Reg<SCK_SPEC>`"]
+pub type SCK = crate::Reg<sck::SCK_SPEC>;
+#[doc = "Pin select for SCK"]
+pub mod sck;
+#[doc = "MOSI (rw) register accessor: an alias for `Reg<MOSI_SPEC>`"]
+pub type MOSI = crate::Reg<mosi::MOSI_SPEC>;
+#[doc = "Pin select for MOSI"]
+pub mod mosi;
+#[doc = "MISO (rw) register accessor: an alias for `Reg<MISO_SPEC>`"]
+pub type MISO = crate::Reg<miso::MISO_SPEC>;
+#[doc = "Pin select for MISO"]
+pub mod miso;