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-executor/tests/ui.rs

@@ -0,0 +1,23 @@
+#[cfg(not(miri))]
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/abi.rs");
+    t.compile_fail("tests/ui/bad_return.rs");
+    t.compile_fail("tests/ui/generics.rs");
+    t.compile_fail("tests/ui/impl_trait_nested.rs");
+    t.compile_fail("tests/ui/impl_trait.rs");
+    t.compile_fail("tests/ui/impl_trait_static.rs");
+    t.compile_fail("tests/ui/nonstatic_ref_anon_nested.rs");
+    t.compile_fail("tests/ui/nonstatic_ref_anon.rs");
+    t.compile_fail("tests/ui/nonstatic_ref_elided.rs");
+    t.compile_fail("tests/ui/nonstatic_ref_generic.rs");
+    t.compile_fail("tests/ui/nonstatic_struct_anon.rs");
+    #[cfg(not(feature = "nightly"))] // we can't catch this case with the macro, so the output changes on nightly.
+    t.compile_fail("tests/ui/nonstatic_struct_elided.rs");
+    t.compile_fail("tests/ui/nonstatic_struct_generic.rs");
+    t.compile_fail("tests/ui/not_async.rs");
+    t.compile_fail("tests/ui/self_ref.rs");
+    t.compile_fail("tests/ui/self.rs");
+    t.compile_fail("tests/ui/where_clause.rs");
+}