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/proc-macro-error2/.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+on:
+  pull_request:
+  push:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust-version: ["stable", "beta", "nightly"]
+
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup toolchain install ${{ matrix.rust-version }}
+      - run: cargo +${{ matrix.rust-version }} test --all
+
+  msrv-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup toolchain install 1.61
+      - run: cargo +1.61 check --all
+
+  ui-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: RUSTFLAGS='--cfg run_ui_tests' cargo +stable test ui
+
+  test-fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: cargo fmt --check --all