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/winnow/examples/css/parser.rs

@@ -0,0 +1,36 @@
+use winnow::combinator::seq;
+use winnow::prelude::*;
+use winnow::token::take_while;
+use winnow::Result;
+
+#[derive(Debug, Eq, PartialEq)]
+pub(crate) struct Color {
+    pub(crate) red: u8,
+    pub(crate) green: u8,
+    pub(crate) blue: u8,
+}
+
+impl std::str::FromStr for Color {
+    // The error must be owned
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        hex_color.parse(s).map_err(|e| e.to_string())
+    }
+}
+
+pub(crate) fn hex_color(input: &mut &str) -> Result<Color> {
+    seq!(Color {
+        _: '#',
+        red: hex_primary,
+        green: hex_primary,
+        blue: hex_primary
+    })
+    .parse_next(input)
+}
+
+fn hex_primary(input: &mut &str) -> Result<u8> {
+    take_while(2, |c: char| c.is_ascii_hexdigit())
+        .try_map(|input| u8::from_str_radix(input, 16))
+        .parse_next(input)
+}