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/defmt/tests/ui/derive-invalid-crate-overwrite.stderr

@@ -0,0 +1,25 @@
+error: expected identifier
+ --> tests/ui/derive-invalid-crate-overwrite.rs:6:17
+  |
+6 | #[defmt(crate = "not a path")]
+  |                 ^^^^^^^^^^^^
+
+error: expected `=`
+  --> tests/ui/derive-invalid-crate-overwrite.rs:10:14
+   |
+10 | #[defmt(crate(defmt))]
+   |              ^
+
+error[E0432]: unresolved import `unresolved`
+ --> tests/ui/derive-invalid-crate-overwrite.rs:1:10
+  |
+1 | #[derive(defmt::Format)]
+  |          ^^^^^^^^^^^^^ no external crate `unresolved`
+  |
+  = note: this error originates in the derive macro `defmt::Format` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0433]: failed to resolve: use of undeclared crate or module `unresolved`
+ --> tests/ui/derive-invalid-crate-overwrite.rs:2:17
+  |
+2 | #[defmt(crate = unresolved)]
+  |                 ^^^^^^^^^^ use of undeclared crate or module `unresolved`