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/embedded-hal-nb/Cargo.toml

@@ -0,0 +1,46 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2021"
+rust-version = "1.56"
+name = "embedded-hal-nb"
+version = "1.0.0"
+description = "Non-blocking Hardware Abstraction Layer (HAL) for embedded systems using the `nb` crate."
+documentation = "https://docs.rs/embedded-hal-nb"
+readme = "README.md"
+keywords = [
+    "hal",
+    "IO",
+]
+categories = [
+    "embedded",
+    "hardware-support",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-embedded/embedded-hal"
+
+[dependencies.embedded-hal]
+version = "1.0.0"
+
+[dependencies.nb]
+version = "1"
+
+[dev-dependencies.cortex-m-rt]
+version = "0.7"
+
+[dev-dependencies.stm32f1]
+version = "0.15"
+features = [
+    "stm32f103",
+    "rt",
+]