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/thiserror/tests/ui/missing-display.stderr

@@ -0,0 +1,19 @@
+error[E0277]: `MyError` doesn't implement `std::fmt::Display`
+ --> tests/ui/missing-display.rs:4:10
+  |
+3 | #[derive(Error, Debug)]
+  |          ----- in this derive macro expansion
+4 | pub enum MyError {
+  |          ^^^^^^^ unsatisfied trait bound
+  |
+help: the trait `std::fmt::Display` is not implemented for `MyError`
+ --> tests/ui/missing-display.rs:4:1
+  |
+4 | pub enum MyError {
+  | ^^^^^^^^^^^^^^^^
+note: required by a bound in `std::error::Error`
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  |                          ^^^^^^^ required by this bound in `Error`
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)