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/minijinja/Cargo.toml

@@ -0,0 +1,151 @@
+# 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 = "1.70"
+name = "minijinja"
+version = "2.18.0"
+authors = ["Armin Ronacher <armin.ronacher@active-4.com>"]
+build = false
+exclude = ["tests"]
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+description = "a powerful template engine for Rust with minimal dependencies"
+homepage = "https://github.com/mitsuhiko/minijinja"
+readme = "README.md"
+keywords = [
+    "jinja",
+    "jinja2",
+    "templates",
+    "template",
+]
+categories = ["template-engine"]
+license = "Apache-2.0"
+repository = "https://github.com/mitsuhiko/minijinja"
+
+[package.metadata.docs.rs]
+features = [
+    "loader",
+    "json",
+    "urlencode",
+    "custom_syntax",
+    "fuel",
+]
+rustdoc-args = [
+    "--cfg",
+    "docsrs",
+    "--html-in-header",
+    "doc-header.html",
+]
+
+[features]
+adjacent_loop_items = []
+builtins = []
+custom_syntax = ["dep:aho-corasick"]
+debug = []
+default = [
+    "builtins",
+    "debug",
+    "deserialization",
+    "macros",
+    "multi_template",
+    "adjacent_loop_items",
+    "std_collections",
+    "serde",
+]
+deserialization = []
+fuel = []
+internal_debug = []
+internal_safe_search = []
+json = ["serde_json"]
+key_interning = []
+loader = []
+loop_controls = []
+macros = []
+multi_template = []
+preserve_order = ["indexmap"]
+serde = []
+speedups = ["v_htmlescape"]
+std_collections = []
+unicode = [
+    "unicode-ident",
+    "unicase",
+]
+unstable_machinery = ["internal_debug"]
+unstable_machinery_serde = [
+    "unstable_machinery",
+    "serde/derive",
+]
+urlencode = ["percent-encoding"]
+
+[lib]
+name = "minijinja"
+path = "src/lib.rs"
+
+[dependencies.aho-corasick]
+version = "1.0"
+optional = true
+default-features = false
+
+[dependencies.indexmap]
+version = "2.2.0"
+optional = true
+
+[dependencies.memo-map]
+version = "0.3.1"
+
+[dependencies.percent-encoding]
+version = "2.2.0"
+optional = true
+
+[dependencies.serde]
+version = "1.0.130"
+
+[dependencies.serde_json]
+version = "1.0.68"
+optional = true
+
+[dependencies.stacker]
+version = "0.1.15"
+optional = true
+
+[dependencies.unicase]
+version = "2.6.0"
+optional = true
+
+[dependencies.unicode-ident]
+version = "1.0.5"
+optional = true
+
+[dependencies.v_htmlescape]
+version = "0.15.8"
+optional = true
+
+[dev-dependencies.insta]
+version = "1.38.0"
+features = [
+    "glob",
+    "serde",
+]
+
+[dev-dependencies.serde]
+version = "1.0.130"
+features = ["derive"]
+
+[dev-dependencies.serde_json]
+version = "1.0.68"
+
+[dev-dependencies.similar-asserts]
+version = "1.4.2"