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/byte-slice-cast/Cargo.toml

@@ -0,0 +1,45 @@
+# 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 = "2018"
+rust-version = "1.51.0"
+name = "byte-slice-cast"
+version = "1.2.3"
+authors = ["Sebastian Dröge <sebastian@centricular.com>"]
+build = false
+include = [
+    "src/lib.rs",
+    "Cargo.toml",
+    "LICENSE",
+    "README.md",
+    "CHANGELOG.md",
+]
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Safely cast bytes slices from/to slices of built-in fundamental numeric types"
+readme = "README.md"
+keywords = ["no_std"]
+license = "MIT"
+repository = "https://github.com/sdroege/bytes-num-slice-cast"
+
+[features]
+default = ["std"]
+std = []
+
+[lib]
+name = "byte_slice_cast"
+path = "src/lib.rs"
+
+[dependencies]