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/i2c_interface.rs

@@ -0,0 +1,64 @@
+//! I2C interface factory
+
+use display_interface_i2c::I2CInterface;
+
+/// Helper struct to create preconfigured I2C interfaces for the display.
+#[derive(Debug, Copy, Clone)]
+pub struct I2CDisplayInterface(());
+
+impl I2CDisplayInterface {
+    /// Create new builder with a default I2C address of 0x3C
+    #[cfg(not(feature = "async"))]
+    #[allow(clippy::new_ret_no_self)]
+    // pub fn with_i2c<I>(i2c: I) -> I2CInterface<I> // alternative, but breaking change
+    pub fn new<I>(i2c: I) -> I2CInterface<I>
+    where
+        I: embedded_hal::i2c::I2c,
+    {
+        Self::new_custom_address(i2c, 0x3C)
+    }
+    #[cfg(feature = "async")]
+    #[allow(clippy::new_ret_no_self)]
+    /// Create a new async I2C interface with the address 0x3D
+    pub fn new<I>(i2c: I) -> I2CInterface<I>
+    where
+        I: embedded_hal_async::i2c::I2c,
+    {
+        Self::new_custom_address(i2c, 0x3C)
+    }
+
+    #[cfg(not(feature = "async"))]
+    /// Create a new I2C interface with the alternate address 0x3D as specified in the datasheet.
+    pub fn new_alternate_address<I>(i2c: I) -> I2CInterface<I>
+    where
+        I: embedded_hal::i2c::I2c,
+    {
+        Self::new_custom_address(i2c, 0x3D)
+    }
+
+    #[cfg(feature = "async")]
+    /// Create a new async I2C interface with the alternate address 0x3D as specified in the datasheet.
+    pub fn new_alternate_address<I>(i2c: I) -> I2CInterface<I>
+    where
+        I: embedded_hal_async::i2c::I2c,
+    {
+        Self::new_custom_address(i2c, 0x3D)
+    }
+
+    #[cfg(not(feature = "async"))]
+    /// Create a new I2C interface with a custom address.
+    pub fn new_custom_address<I>(i2c: I, address: u8) -> I2CInterface<I>
+    where
+        I: embedded_hal::i2c::I2c,
+    {
+        I2CInterface::new(i2c, address, 0x40)
+    }
+    #[cfg(feature = "async")]
+    /// Create a new  async I2C interface with a custom address.
+    pub fn new_custom_address<I>(i2c: I, address: u8) -> I2CInterface<I>
+    where
+        I: embedded_hal_async::i2c::I2c,
+    {
+        I2CInterface::new(i2c, address, 0x40)
+    }
+}