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-sync-0.6.2/Cargo.toml

@@ -0,0 +1,98 @@
+# 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 = "embassy-sync"
+version = "0.6.2"
+build = "build.rs"
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "no-std, no-alloc synchronization primitives with async support"
+documentation = "https://docs.embassy.dev/embassy-sync"
+readme = "README.md"
+categories = [
+    "embedded",
+    "no-std",
+    "concurrency",
+    "asynchronous",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/embassy-rs/embassy"
+
+[package.metadata.embassy_docs]
+src_base = "https://github.com/embassy-rs/embassy/blob/embassy-sync-v$VERSION/embassy-sync/src/"
+src_base_git = "https://github.com/embassy-rs/embassy/blob/$COMMIT/embassy-sync/src/"
+target = "thumbv7em-none-eabi"
+
+[lib]
+name = "embassy_sync"
+path = "src/lib.rs"
+
+[dependencies.cfg-if]
+version = "1.0.0"
+
+[dependencies.critical-section]
+version = "1.1"
+
+[dependencies.defmt]
+version = "0.3"
+optional = true
+
+[dependencies.embedded-io-async]
+version = "0.6.1"
+
+[dependencies.futures-sink]
+version = "0.3"
+features = []
+default-features = false
+
+[dependencies.futures-util]
+version = "0.3.17"
+default-features = false
+
+[dependencies.heapless]
+version = "0.8"
+
+[dependencies.log]
+version = "0.4.14"
+optional = true
+
+[dev-dependencies.critical-section]
+version = "1.1"
+features = ["std"]
+
+[dev-dependencies.futures-executor]
+version = "0.3.17"
+features = ["thread-pool"]
+
+[dev-dependencies.futures-test]
+version = "0.3.17"
+
+[dev-dependencies.futures-timer]
+version = "3.0.2"
+
+[dev-dependencies.futures-util]
+version = "0.3.17"
+features = [
+    "channel",
+    "sink",
+]
+
+[dev-dependencies.static_cell]
+version = "2"
+
+[features]
+std = ["critical-section/std"]
+turbowakers = []