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.rs

@@ -0,0 +1,42 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0400],
+    #[doc = "0x400..0x488 - Unspecified"]
+    pub mailbox: MAILBOX,
+    _reserved1: [u8; 0x78],
+    #[doc = "0x500..0x508 - Unspecified"]
+    pub eraseprotect: ERASEPROTECT,
+    _reserved2: [u8; 0x38],
+    #[doc = "0x540..0x548 - Unspecified"]
+    pub approtect: APPROTECT,
+    #[doc = "0x548..0x550 - Unspecified"]
+    pub secureapprotect: SECUREAPPROTECT,
+    _reserved4: [u8; 0xb0],
+    #[doc = "0x600 - Status bits for CTRL-AP peripheral."]
+    pub status: STATUS,
+}
+#[doc = "Unspecified"]
+pub use mailbox::MAILBOX;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod mailbox;
+#[doc = "Unspecified"]
+pub use eraseprotect::ERASEPROTECT;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod eraseprotect;
+#[doc = "Unspecified"]
+pub use approtect::APPROTECT;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod approtect;
+#[doc = "Unspecified"]
+pub use secureapprotect::SECUREAPPROTECT;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod secureapprotect;
+#[doc = "STATUS (r) register accessor: an alias for `Reg<STATUS_SPEC>`"]
+pub type STATUS = crate::Reg<status::STATUS_SPEC>;
+#[doc = "Status bits for CTRL-AP peripheral."]
+pub mod status;