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/anyhow/tests/ui/no-impl.stderr

@@ -0,0 +1,32 @@
+error[E0599]: the method `anyhow_kind` exists for reference `&Error`, but its trait bounds were not satisfied
+ --> tests/ui/no-impl.rs:7:13
+  |
+4 | struct Error;
+  | ------------ doesn't satisfy `Error: Into<anyhow::Error>`, `Error: anyhow::kind::TraitKind` or `Error: std::fmt::Display`
+...
+7 |     let _ = anyhow!(Error);
+  |             ^^^^^^^^^^^^^^ method cannot be called on `&Error` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `Error: Into<anyhow::Error>`
+          which is required by `Error: anyhow::kind::TraitKind`
+          `Error: std::fmt::Display`
+          which is required by `&Error: anyhow::kind::AdhocKind`
+          `&Error: Into<anyhow::Error>`
+          which is required by `&Error: anyhow::kind::TraitKind`
+note: the traits `Into` and `std::fmt::Display` must be implemented
+ --> $RUST/core/src/fmt/mod.rs
+  |
+  | pub trait Display: PointeeSized {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+ ::: $RUST/core/src/convert/mod.rs
+  |
+  | pub const trait Into<T>: Sized {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following traits define an item `anyhow_kind`, perhaps you need to implement one of them:
+          candidate #1: `anyhow::kind::AdhocKind`
+          candidate #2: `anyhow::kind::BoxedKind`
+          candidate #3: `anyhow::kind::TraitKind`
+  = note: this error originates in the macro `anyhow` (in Nightly builds, run with -Z macro-backtrace for more info)