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/toml/src/table.rs

@@ -0,0 +1,115 @@
+use serde::de;
+use serde::ser;
+
+use crate::map::Map;
+use crate::Value;
+
+/// Type representing a TOML table, payload of the `Value::Table` variant.
+///
+/// By default it entries are stored in
+/// [lexicographic order](https://doc.rust-lang.org/std/primitive.str.html#impl-Ord-for-str)
+/// of the keys. Enable the `preserve_order` feature to store entries in the order they appear in
+/// the source file.
+pub type Table = Map<String, Value>;
+
+impl Table {
+    /// Convert a `T` into `toml::Table`.
+    ///
+    /// This conversion can fail if `T`'s implementation of `Serialize` decides to
+    /// fail, or if `T` contains a map with non-string keys.
+    pub fn try_from<T>(value: T) -> Result<Self, crate::ser::Error>
+    where
+        T: ser::Serialize,
+    {
+        value.serialize(crate::value::TableSerializer)
+    }
+
+    /// Interpret a `toml::Table` as an instance of type `T`.
+    ///
+    /// This conversion can fail if the structure of the `Table` does not match the structure
+    /// expected by `T`, for example if `T` is a bool which can't be mapped to a `Table`. It can
+    /// also fail if the structure is correct but `T`'s implementation of `Deserialize` decides
+    /// that something is wrong with the data, for example required struct fields are missing from
+    /// the TOML map or some number is too big to fit in the expected primitive type.
+    pub fn try_into<'de, T>(self) -> Result<T, crate::de::Error>
+    where
+        T: de::Deserialize<'de>,
+    {
+        de::Deserialize::deserialize(self)
+    }
+}
+
+#[cfg(feature = "display")]
+impl std::fmt::Display for Table {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        crate::ser::to_string(self)
+            .expect("Unable to represent value as string")
+            .fmt(f)
+    }
+}
+
+#[cfg(feature = "parse")]
+impl std::str::FromStr for Table {
+    type Err = crate::de::Error;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        crate::from_str(s)
+    }
+}
+
+impl<'de> de::Deserializer<'de> for Table {
+    type Error = crate::de::Error;
+
+    fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, crate::de::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        Value::Table(self).deserialize_any(visitor)
+    }
+
+    #[inline]
+    fn deserialize_enum<V>(
+        self,
+        name: &'static str,
+        variants: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, crate::de::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        Value::Table(self).deserialize_enum(name, variants, visitor)
+    }
+
+    // `None` is interpreted as a missing field so be sure to implement `Some`
+    // as a present field.
+    fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, crate::de::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        Value::Table(self).deserialize_option(visitor)
+    }
+
+    fn deserialize_newtype_struct<V>(
+        self,
+        name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, crate::de::Error>
+    where
+        V: de::Visitor<'de>,
+    {
+        Value::Table(self).deserialize_newtype_struct(name, visitor)
+    }
+
+    serde::forward_to_deserialize_any! {
+        bool u8 u16 u32 u64 i8 i16 i32 i64 f32 f64 char str string unit seq
+        bytes byte_buf map unit_struct tuple_struct struct
+        tuple ignored_any identifier
+    }
+}
+
+impl de::IntoDeserializer<'_, crate::de::Error> for Table {
+    type Deserializer = Self;
+
+    fn into_deserializer(self) -> Self {
+        self
+    }
+}