ci(demo): add workflow rendering every card for every theme (#1)

Runs on push to main (skipping its own demo/output/markdown commits),
builds the ghstats binary, renders every theme for the repo owner,
and writes a demo/README.md gallery back to main.

.github/workflows/demo.yml

@@ -0,0 +1,102 @@
+name: Demo
+
+on:
+  push:
+    branches: [main]
+    paths-ignore:
+      - "demo/**"
+      - "output/**"
+      - "**.md"
+      - "LICENSE"
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+concurrency:
+  group: demo
+  cancel-in-progress: true
+
+jobs:
+  generate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-go@v6
+        with:
+          go-version: "1.26"
+          cache: true
+
+      - name: Build ghstats
+        run: go build -trimpath -ldflags="-s -w" -o ghstats .
+
+      - name: Render every card for every theme
+        env:
+          GITHUB_TOKEN: ${{ secrets.GHSTATS_TOKEN }}
+        run: |
+          rm -rf demo
+          mkdir -p demo
+          ./ghstats \
+            -user "${{ github.repository_owner }}" \
+            -themes all \
+            -tz Asia/Saigon \
+            -out demo
+
+      - name: Write demo README
+        env:
+          OWNER: ${{ github.repository_owner }}
+          REPO: ${{ github.repository }}
+        run: |
+          cards=(
+            profile-details
+            repos-per-language
+            most-commit-language
+            stats
+            productive-time
+            contributions
+            most-commit-language-all-time
+            productive-time-all-time
+            contributions-all-time
+          )
+          themes=$(./ghstats -list-themes)
+          {
+            echo "# ghstats demo gallery"
+            echo
+            echo "Every card in every theme, rendered for [\`$OWNER\`](https://github.com/$OWNER)."
+            echo
+            echo "Auto-generated by [\`.github/workflows/demo.yml\`](../.github/workflows/demo.yml) on each push to \`main\`. Do not edit by hand."
+            echo
+            echo "## Themes"
+            echo
+            for theme in $themes; do
+              echo "- [\`$theme\`](#$theme)"
+            done
+            echo
+            for theme in $themes; do
+              dir="demo/$theme"
+              [ -d "$dir" ] || continue
+              echo "## $theme"
+              echo
+              for card in "${cards[@]}"; do
+                svg="$dir/$card.svg"
+                [ -f "$svg" ] || continue
+                echo "![$card](./$theme/$card.svg)"
+              done
+              echo
+              echo "[back to top](#themes)"
+              echo
+            done
+          } > demo/README.md
+
+      - name: Commit demo back to main
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git add demo
+          if git diff --cached --quiet; then
+            echo "No demo changes to commit."
+            exit 0
+          fi
+          git commit -m "chore(demo): regenerate gallery"
+          git push origin HEAD:${{ github.ref_name }}