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/duplicate-fmt.stderr

@@ -0,0 +1,23 @@
+error: only one #[error(...)] attribute is allowed
+ --> tests/ui/duplicate-fmt.rs:5:1
+  |
+5 | #[error("...")]
+  | ^^^^^^^^^^^^^^^
+
+error: duplicate #[error(fmt = ...)] attribute
+  --> tests/ui/duplicate-fmt.rs:10:1
+   |
+10 | #[error(fmt = core::fmt::LowerHex::fmt)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: cannot have both #[error(transparent)] and #[error(fmt = ...)]
+  --> tests/ui/duplicate-fmt.rs:14:1
+   |
+14 | #[error(fmt = core::fmt::Octal::fmt)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: cannot have both #[error(fmt = ...)] and a format arguments attribute
+  --> tests/ui/duplicate-fmt.rs:20:1
+   |
+20 | #[error("...")]
+   | ^^^^^^^^^^^^^^^