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/pin-project-lite/tests/ui/pinned_drop/call-drop-inner.stderr

@@ -0,0 +1,22 @@
+error[E0061]: this function takes 0 arguments but 1 argument was supplied
+  --> tests/ui/pinned_drop/call-drop-inner.rs:12:13
+   |
+12 |             __drop_inner(this);
+   |             ^^^^^^^^^^^^ ---- unexpected argument of type `Pin<&mut S>`
+   |
+note: function defined here
+  --> tests/ui/pinned_drop/call-drop-inner.rs:5:1
+   |
+ 5 | / pin_project! {
+ 6 | |     pub struct S {
+ 7 | |         #[pin]
+ 8 | |         field: u8,
+...  |
+15 | | }
+   | |_^
+   = note: this error originates in the macro `$crate::__pin_project_make_drop_impl` which comes from the expansion of the macro `pin_project` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: remove the extra argument
+   |
+12 -             __drop_inner(this);
+12 +             __drop_inner();
+   |