feat: Add HTML export functionality and GitHub workflow

- Implement HTMLExporter with professional styling and embedded CSS
- Add comprehensive test suite for HTML export functionality
- Update factory to support HTML format ('html' and 'htm')
- Add autofix.ci GitHub workflow for code formatting
- Support all content types: text, lists, quizzes, multimedia, etc.
- Include proper HTML escaping for security
- Add benchmark tests for performance validation

.github/workflows/autofix.yml

@@ -0,0 +1,25 @@
+name: autofix.ci
+on:
+  pull_request:
+  push:
+    branches: [ "master" ]
+permissions:
+  contents: read
+
+jobs:
+  autofix:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: 'go.mod'
+
+      # goimports works like gofmt, but also fixes imports.
+      # see https://pkg.go.dev/golang.org/x/tools/cmd/goimports
+      - run: go install golang.org/x/tools/cmd/goimports@latest
+      - run: goimports -w .
+      # of course we can also do just this instead:
+      # - run: gofmt -w .
+
+      - uses: autofix-ci/action@551dded8c6cc8a1054039c8bc0b8b48c51dfc6ef