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/chrono/CITATION.cff

@@ -0,0 +1,33 @@
+# Parser settings.
+cff-version: 1.2.0
+message: Please cite this crate using these information.
+
+# Version information.
+date-released: 2026-01-09
+version: 0.4.43
+
+# Project information.
+abstract: Date and time library for Rust
+authors:
+  - alias: quodlibetor
+    family-names: Maister
+    given-names: Brandon W.
+  - alias: djc
+    family-names: Ochtman
+    given-names: Dirkjan
+  - alias: lifthrasiir
+    family-names: Seonghoon
+    given-names: Kang
+  - alias: esheppa
+    family-names: Sheppard
+    given-names: Eric
+  - alias: pitdicker
+    family-names: Dicker
+    given-names: Paul
+license:
+  - Apache-2.0
+  - MIT
+repository-artifact: https://crates.io/crates/chrono
+repository-code: https://github.com/chronotope/chrono
+title: chrono
+url: https://docs.rs/chrono