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/source-enum-not-error.stderr

@@ -0,0 +1,22 @@
+error[E0599]: the method `as_dyn_error` exists for reference `&NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-enum-not-error.rs:9:14
+  |
+4 | pub struct NotError;
+  | ------------------- doesn't satisfy `NotError: AsDynError<'_>` or `NotError: std::error::Error`
+...
+9 |     Broken { source: NotError },
+  |              ^^^^^^ method cannot be called on `&NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+          `&NotError: std::error::Error`
+          which is required by `&NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following trait defines an item `as_dyn_error`, perhaps you need to implement it:
+          candidate #1: `AsDynError`