Add GitHub Actions: CI and a manual release workflow

- ci.yml: runs the unit test suite (npm test → node --test tests/unit) on
  push and PR to main, across ubuntu/windows/macos with Node 20. Electron is
  installed normally because the capture unit tests require app/capture.js,
  which require()s the electron module at load.

- release.yml: manual workflow_dispatch. Enter a version tag (e.g. v0.1.1);
  it builds the Windows portable .exe (npm run package:windows) and the Linux
  tarball + .deb (scripts/package-linux.sh), stamps the requested version onto
  each artifact, then publishes a GitHub Release at the current commit with
  the artifacts attached and auto-generated notes. Supports a prerelease flag.

.github/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+# Cancel an in-progress run when newer commits are pushed to the same ref.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: Unit tests (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      # The capture unit tests require app/capture.js, which require()s the
+      # electron module at load — so the Electron binary must actually be
+      # installed (don't set ELECTRON_SKIP_BINARY_DOWNLOAD here).
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run unit tests
+        run: npm test