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-graphics-core/Cargo.toml

@@ -0,0 +1,74 @@
+# 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"
+name = "embedded-graphics-core"
+version = "0.4.1"
+authors = [
+    "James Waples <james@wapl.es>",
+    "Ralf Fuest <mail@rfuest.de>",
+]
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Core traits and functionality for embedded-graphics"
+documentation = "https://docs.rs/embedded-graphics-core"
+readme = "README.md"
+keywords = [
+    "embedded-graphics",
+    "graphics",
+    "embedded",
+]
+categories = [
+    "embedded",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/embedded-graphics/embedded-graphics"
+
+[package.metadata.docs.rs]
+all-features = true
+
+[badges.circle-ci]
+branch = "master"
+repository = "embedded-graphics/embedded-graphics"
+
+[features]
+default = []
+nalgebra_support = ["nalgebra"]
+
+[lib]
+name = "embedded_graphics_core"
+path = "src/lib.rs"
+
+[dependencies.az]
+version = "~1.2.0"
+
+[dependencies.byteorder]
+version = "1.3.4"
+default-features = false
+
+[dependencies.defmt]
+version = "0.3.2"
+optional = true
+
+[dependencies.nalgebra]
+version = "0.30.1"
+optional = true
+default-features = false
+
+[dev-dependencies.arrayvec]
+version = "0.7.2"
+default-features = false