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/quote/tests/ui/not-quotable.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the trait bound `Ipv4Addr: ToTokens` is not satisfied
+ --> tests/ui/not-quotable.rs:6:13
+  |
+6 |     let _ = quote! { #ip };
+  |             ^^^^^^^^^^^^^^
+  |             |
+  |             the trait `ToTokens` is not implemented for `Ipv4Addr`
+  |             required by a bound introduced by this call
+  |
+  = help: the following other types implement trait `ToTokens`:
+            &T
+            &mut T
+            Arc<T>
+            Box<T>
+            CStr
+            CString
+            Cow<'a, T>
+            Option<T>
+          and $N others
+  = note: this error originates in the macro `quote` (in Nightly builds, run with -Z macro-backtrace for more info)