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/fugit/README.md

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+# `fugit`
+
+> `fugit` provides a comprehensive library of `Duration` and `Instant` for the handling of time in embedded systems, doing all it can at compile time.
+
+This library is a heavily inspired of `std::chrono`'s `Duration` from C++ which does all it can at compile time.
+
+## Aims
+
+* `no_std` library with goals of user-friendliness and performance first
+  * All methods are `const fn` that can be (i.e. non-trait methods)
+  * Use no traits, concrete types all the way for maximum `const`-ification
+  * Operations are supported between different bases and backing storages instead of implementing custom traits
+  * All constants needed for comparing or changing timebase are guaranteed compile time generated
+* Support for both `u32` and `u64` backing storage with efficient instruction lowering on MCUs
+  * On Cortex-M3 and up: no soft-impls pulled in for both `u32` and `u64` except when changing base on `u64`
+  * Comparisons on `u32` and `u64` do not use division, only changing base with all constants calculated at compile time
+* Selection of base happens at compile time
+  * A common problem is that run time changing of base robs us of a lot of optimization opportunities, but since there are no traits and short-hands select the correct base at compile time.
+