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/proc-macro-error2/src/imp/fallback.rs

@@ -0,0 +1,30 @@
+//! This implementation uses self-written stable facilities.
+
+use crate::{
+    abort_now, check_correctness,
+    diagnostic::{Diagnostic, Level},
+};
+use std::cell::RefCell;
+
+pub fn abort_if_dirty() {
+    check_correctness();
+    ERR_STORAGE.with(|storage| {
+        if !storage.borrow().is_empty() {
+            abort_now()
+        }
+    });
+}
+
+pub(crate) fn cleanup() -> Vec<Diagnostic> {
+    ERR_STORAGE.with(|storage| storage.replace(Vec::new()))
+}
+
+pub(crate) fn emit_diagnostic(diag: Diagnostic) {
+    if diag.level == Level::Error {
+        ERR_STORAGE.with(|storage| storage.borrow_mut().push(diag));
+    }
+}
+
+thread_local! {
+    static ERR_STORAGE: RefCell<Vec<Diagnostic>> = const { RefCell::new(Vec::new()) };
+}