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/proc-macro-error-attr2/Cargo.toml

@@ -0,0 +1,44 @@
+# 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 = "1.61"
+name = "proc-macro-error-attr2"
+version = "2.0.0"
+authors = [
+    "CreepySkeleton <creepy-skeleton@yandex.ru>",
+    "GnomedDev <david2005thomas@gmail.com>",
+]
+build = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "Attribute macro for the proc-macro-error2 crate"
+readme = false
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/GnomedDev/proc-macro-error-2"
+
+[lib]
+name = "proc_macro_error_attr2"
+path = "src/lib.rs"
+proc-macro = true
+
+[dependencies.proc-macro2]
+version = "1"
+
+[dependencies.quote]
+version = "1"
+
+[lints.clippy.pedantic]
+level = "warn"
+priority = -1