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/embassy-time/README.md

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+# embassy-time
+
+Timekeeping, delays and timeouts.
+
+Timekeeping is done with elapsed time since system boot. Time is represented in
+ticks, where the tick rate is defined either by the driver (in the case of a fixed-rate
+tick) or chosen by the user with a [tick rate](#tick-rate) feature. The chosen
+tick rate applies to everything in `embassy-time` and thus determines the maximum
+timing resolution of <code>(1 / tick_rate) seconds</code>.
+
+Tick counts are 64 bits. The default tick rate of 1Mhz supports
+representing time spans of up to ~584558 years, which is big enough for all practical
+purposes and allows not having to worry about overflows.
+
+## Global time driver
+
+The `time` module is backed by a global "time driver" specified at build time.
+Only one driver can be active in a program.
+
+All methods and structs transparently call into the active driver. This makes it
+possible for libraries to use `embassy_time` in a driver-agnostic way without
+requiring generic parameters.
+
+For more details, check the [`embassy_time_driver`](https://crates.io/crates/embassy-time-driver) crate.
+
+## Instants and Durations
+
+[`Instant`] represents a given instant of time (relative to system boot), and [`Duration`]
+represents the duration of a span of time. They implement the math operations you'd expect,
+like addition and substraction.
+
+## Delays and timeouts
+
+[`Timer`] allows performing async delays. [`Ticker`] allows periodic delays without drifting over time.
+
+An implementation of the `embedded-hal` delay traits is provided by [`Delay`], for compatibility
+with libraries from the ecosystem.
+
+## Wall-clock time
+
+The `time` module deals exclusively with a monotonically increasing tick count.
+Therefore it has no direct support for wall-clock time ("real life" datetimes
+like `2021-08-24 13:33:21`).
+
+If persistence across reboots is not needed, support can be built on top of
+`embassy_time` by storing the offset between "seconds elapsed since boot"
+and "seconds since unix epoch".