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-storage-async/Cargo.toml

@@ -0,0 +1,36 @@
+# 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"
+name = "embedded-storage-async"
+version = "0.4.1"
+authors = [
+    "Mathias Koch <mk@blackbird.online>",
+    "Ulf Lilleengen <lulf@redhat.com>",
+    "Dario Nieuwenhuis <dirbaio@dirbaio.net>",
+    "Diego Barrios Romero <eldruin@gmail.com>",
+]
+description = "A Storage Abstraction Layer for Embedded Systems"
+homepage = "https://github.com/rust-embedded-community/embedded-storage"
+documentation = "https://docs.rs/embedded-storage"
+readme = "README.md"
+keywords = ["storage"]
+categories = [
+    "embedded",
+    "hardware-support",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-embedded-community/embedded-storage"
+
+[dependencies.embedded-storage]
+version = "0.3.1"