ci: add GitHub Actions workflow for desktop release builds

Triggers on v* tag push, builds for Windows, macOS (arm64), and Linux.
Uses electron-builder --publish to upload artifacts to GitHub Releases.

.github/workflows/release.yml

@@ -0,0 +1,50 @@
+name: Release Desktop
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+permissions:
+  contents: write
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        include:
+          - os: windows-latest
+            platform: windows
+          - os: ubuntu-latest
+            platform: linux
+          - os: macos-latest
+            platform: mac
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Build shared package
+        run: pnpm --filter @backspace/shared build
+
+      - name: Build desktop app
+        working-directory: packages/desktop
+        run: pnpm run build
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}