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/postcard/tests/accumulator.rs

@@ -0,0 +1,59 @@
+use postcard::accumulator::{CobsAccumulator, FeedResult};
+use serde::{Deserialize, Serialize};
+use std::io::Read;
+
+// Read a "huge" serialized struct in 32 byte chunks into a 256 byte buffer and deserialize it.
+#[test]
+fn reader() {
+    let mut raw_buf = [0u8; 32];
+    let mut input_buf = [0u8; 256];
+    let mut cobs_buf: CobsAccumulator<256> = CobsAccumulator::new();
+
+    #[derive(Serialize, Deserialize, Debug, PartialEq, Eq)]
+    struct Huge {
+        a: u32,
+        b: [u32; 32],
+        d: u64,
+    }
+
+    let expected = Huge {
+        a: 0xabcdef00,
+        b: [0x01234567; 32],
+        d: u64::MAX,
+    };
+
+    let input = postcard::to_slice_cobs(&expected, &mut input_buf).unwrap();
+    // TODO(https://github.com/rust-lang/rust-clippy/issues/12751): Remove once fixed.
+    #[allow(clippy::redundant_slicing)]
+    let mut input = &input[..];
+
+    // Magic number from serializing struct and printing length
+    assert_eq!(input.len(), 145);
+
+    let mut output = None;
+
+    while let Ok(ct) = input.read(&mut raw_buf) {
+        // Finished reading input
+        if ct == 0 {
+            break;
+        }
+
+        let buf = &raw_buf[..ct];
+        let mut window = buf;
+
+        'cobs: while !window.is_empty() {
+            window = match cobs_buf.feed::<Huge>(window) {
+                FeedResult::Consumed => break 'cobs,
+                FeedResult::OverFull(new_wind) => new_wind,
+                FeedResult::DeserError(new_wind) => new_wind,
+                FeedResult::Success { data, remaining } => {
+                    output = Some(data);
+
+                    remaining
+                }
+            };
+        }
+    }
+
+    assert_eq!(output.unwrap(), expected);
+}