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/embassy-sync/tests/ui/sync_impl/lazy_lock_function.rs

@@ -0,0 +1,11 @@
+use embassy_sync::lazy_lock::LazyLock;
+
+fn main() {
+    let x = 128u8;
+    let x_ptr: *const u8 = core::ptr::addr_of!(x);
+    let closure_capturing_non_sync_variable = || unsafe { core::ptr::read(x_ptr) };
+
+    check_sync(LazyLock::new(closure_capturing_non_sync_variable));
+}
+
+fn check_sync<T: Sync>(_lazy_lock: T) {}

vendor/embassy-sync/tests/ui/sync_impl/lazy_lock_function.stderr

@@ -0,0 +1,24 @@
+error[E0277]: `*const u8` cannot be shared between threads safely
+  --> tests/ui/sync_impl/lazy_lock_function.rs:8:16
+   |
+6  |     let closure_capturing_non_sync_variable = || unsafe { core::ptr::read(x_ptr) };
+   |                                               -- within this `{closure@$DIR/tests/ui/sync_impl/lazy_lock_function.rs:6:47: 6:49}`
+7  |
+8  |     check_sync(LazyLock::new(closure_capturing_non_sync_variable));
+   |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*const u8` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: within `{closure@$DIR/tests/ui/sync_impl/lazy_lock_function.rs:6:47: 6:49}`, the trait `Sync` is not implemented for `*const u8`
+   = note: required because it appears within the type `&*const u8`
+note: required because it's used within this closure
+  --> tests/ui/sync_impl/lazy_lock_function.rs:6:47
+   |
+6  |     let closure_capturing_non_sync_variable = || unsafe { core::ptr::read(x_ptr) };
+   |                                               ^^
+   = note: required for `embassy_sync::lazy_lock::LazyLock<u8, {closure@$DIR/tests/ui/sync_impl/lazy_lock_function.rs:6:47: 6:49}>` to implement `Sync`
+note: required by a bound in `check_sync`
+  --> tests/ui/sync_impl/lazy_lock_function.rs:11:18
+   |
+11 | fn check_sync<T: Sync>(_lazy_lock: T) {}
+   |                  ^^^^ required by this bound in `check_sync`

vendor/embassy-sync/tests/ui/sync_impl/lazy_lock_type.rs

@@ -0,0 +1,6 @@
+use embassy_sync::lazy_lock::LazyLock;
+
+// *mut u8 is not Sync, so LazyLock should not implement Sync for this type. This should fail to compile.
+static GLOBAL: LazyLock<*mut u8> = LazyLock::new(|| core::ptr::null_mut());
+
+fn main() {}

vendor/embassy-sync/tests/ui/sync_impl/lazy_lock_type.stderr

@@ -0,0 +1,9 @@
+error[E0277]: `*mut u8` cannot be shared between threads safely
+ --> tests/ui/sync_impl/lazy_lock_type.rs:4:16
+  |
+4 | static GLOBAL: LazyLock<*mut u8> = LazyLock::new(|| core::ptr::null_mut());
+  |                ^^^^^^^^^^^^^^^^^ `*mut u8` cannot be shared between threads safely
+  |
+  = help: the trait `Sync` is not implemented for `*mut u8`
+  = note: required for `embassy_sync::lazy_lock::LazyLock<*mut u8>` to implement `Sync`
+  = note: shared static variables must have a type that implements `Sync`

vendor/embassy-sync/tests/ui/sync_impl/once_lock.rs

@@ -0,0 +1,6 @@
+use embassy_sync::once_lock::OnceLock;
+
+// *mut u8 is not Sync, so OnceLock should not implement Sync for this type. This should fail to compile.
+static GLOBAL: OnceLock<*mut u8> = OnceLock::new();
+
+fn main() {}

vendor/embassy-sync/tests/ui/sync_impl/once_lock.stderr

@@ -0,0 +1,9 @@
+error[E0277]: `*mut u8` cannot be shared between threads safely
+ --> tests/ui/sync_impl/once_lock.rs:4:16
+  |
+4 | static GLOBAL: OnceLock<*mut u8> = OnceLock::new();
+  |                ^^^^^^^^^^^^^^^^^ `*mut u8` cannot be shared between threads safely
+  |
+  = help: the trait `Sync` is not implemented for `*mut u8`
+  = note: required for `embassy_sync::once_lock::OnceLock<*mut u8>` to implement `Sync`
+  = note: shared static variables must have a type that implements `Sync`