feat(ci): add nightly build workflow (#129)

- Adds scheduled daily builds at 2 AM UTC
- Builds Python packages and uploads as artifacts
- 7-day retention to keep storage minimal
- No release tab pollution - artifacts only

.github/workflows/nightly.yml

@@ -0,0 +1,67 @@
+name: Nightly Build
+
+on:
+  schedule:
+    - cron: '0 2 * * *'
+
+permissions:
+  contents: read
+
+jobs:
+  nightly:
+    name: Build and Upload Nightly
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          ref: main
+          fetch-depth: 0
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.10'
+          cache: 'pip'
+
+      - name: Install system dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y \
+            portaudio19-dev \
+            python3-dev \
+            libgirepository1.0-dev \
+            pkg-config
+
+      - name: Install build dependencies
+        run: |
+          python -m pip install --upgrade pip setuptools wheel build twine
+
+      - name: Build package
+        run: |
+          python -m build
+          ls -la dist/
+
+      - name: Generate nightly version
+        id: version
+        run: |
+          NIGHTLY_VERSION=$(grep -oP '__version__\s*=\s*"\K[^"]+' src/vocalinux/version.py)
+          DATE=$(date +%Y%m%d)
+          echo "version=${NIGHTLY_VERSION}.dev${DATE}" >> $GITHUB_OUTPUT
+          echo "Built version: ${NIGHTLY_VERSION}.dev${DATE}"
+
+      - name: Rename artifacts
+        run: |
+          for file in dist/*; do
+            base=$(basename "$file")
+            mv "$file" "dist/nightly-${base#vocalinux-}"
+          done
+          ls -la dist/
+
+      - name: Upload artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: vocalinux-nightly-${{ steps.version.outputs.version }}
+          path: dist/
+          retention-days: 7