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/ssd1306/src/brightness.rs

@@ -0,0 +1,53 @@
+//! Display brightness
+
+/// Struct that holds display brightness
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct Brightness {
+    pub(crate) precharge: u8,
+    pub(crate) contrast: u8,
+}
+
+impl Default for Brightness {
+    fn default() -> Self {
+        Brightness::NORMAL
+    }
+}
+
+impl Brightness {
+    /// The dimmest predefined brightness level
+    pub const DIMMEST: Brightness = Brightness::custom(0x1, 0x00);
+
+    /// A dim predefined brightness level
+    pub const DIM: Brightness = Brightness::custom(0x2, 0x2F);
+
+    /// A medium predefined brightness level
+    pub const NORMAL: Brightness = Brightness::custom(0x2, 0x5F);
+
+    /// A bright predefined brightness level
+    pub const BRIGHT: Brightness = Brightness::custom(0x2, 0x9F);
+
+    /// The brightest predefined brightness level
+    pub const BRIGHTEST: Brightness = Brightness::custom(0x2, 0xFF);
+
+    /// Create a Brightness object from a precharge period and contrast pair.
+    ///
+    /// `precharge` sets the `phase 2` argument of the `0xD9 Set Pre-Charge Period` command and must
+    /// be must be between 1 and 15.
+    /// The effects of this parameter are hardware dependent. For the common 128x64 displays, values
+    /// 1 and 2 result in different brightness levels, values above 2 behave the same was as 2.
+    /// See section 10.1.17 of the SSD1306 datasheet for more information.
+    ///
+    /// `contrast` sets the value used in the `0x81 Set Contrast Control` command and must be
+    /// between 0 and 255. See section 10.1.7 of the SSD1306 datasheet for more information.
+    pub const fn custom(precharge: u8, contrast: u8) -> Self {
+        debug_assert!(
+            0 < precharge && precharge <= 15,
+            "Precharge value must be between 1 and 15"
+        );
+
+        Self {
+            precharge,
+            contrast,
+        }
+    }
+}