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/thiserror/tests/ui/unconditional-recursion.stderr

@@ -0,0 +1,21 @@
+error[E0425]: cannot find value `__FAIL__` in this scope
+ --> tests/ui/unconditional-recursion.rs:8:5
+  |
+8 |     __FAIL__;
+  |     ^^^^^^^^ not found in this scope
+
+warning: function cannot return without recursing
+ --> tests/ui/unconditional-recursion.rs:4:9
+  |
+4 | #[error("{self}")]
+  |         ^^^^^^^^
+  |         |
+  |         cannot return without recursing
+  |         recursive call site
+  |
+  = help: a `loop` may express intention better if this is on purpose
+note: the lint level is defined here
+ --> tests/ui/unconditional-recursion.rs:4:9
+  |
+4 | #[error("{self}")]
+  |         ^^^^^^^^