feat: Additive merge of webui branch with PyQt GUI support

- Add abogen/pyqt/ package with full PyQt6 desktop GUI
- Restore PyQt GUI files (gui.py, book_handler.py, queue_manager_gui.py, voice_formula_gui.py, conversion.py)
- Add new shared modules from webui: book_parser.py, subtitle_utils.py, spacy_utils.py
- Add Linux libraries (libxcb-cursor) for Qt platform plugin support
- Add new epub parsing tests from webui branch
- Update pyproject.toml with dual entry points:
  - abogen: PyQt6 desktop GUI
  - abogen-web: Flask Web UI
- Add PyQt6 to dependencies
- Re-export PyQt classes from root modules for backwards compatibility
- Merge CHANGELOG.md entries (1.2.0-1.2.5 from webui)
- Update README.md with dual interface documentation

Implements #26 - shared core with separate UI folders

build_pypi.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""Build PyPI package (wheel and sdist) to `dist` folder for abogen."""
+
+import subprocess
+import os
+import shutil
+import tempfile
+
+
+def main():
+    script_dir = os.path.dirname(os.path.abspath(__file__))
+    output_dir = os.path.join(script_dir, "dist")
+
+    print("🔧 abogen PyPI Package Builder")
+    print("=" * 40)
+    print(f"📁 Script directory: {script_dir}")
+    print(f"📦 Output directory: {output_dir}")
+
+    # Try to print package version if present
+    version = None
+    version_file = os.path.join(script_dir, "abogen", "VERSION")
+    if os.path.isfile(version_file):
+        try:
+            with open(version_file, "r", encoding="utf-8") as vf:
+                version = vf.read().strip()
+        except Exception:
+            version = None
+    if version:
+        print(f"🔖 Package version: {version}")
+    
+    # Check if build module is installed, install if not
+    # Temporarily remove script_dir from sys.path to avoid importing local build.py
+    import sys
+    original_path = sys.path[:]
+    try:
+        sys.path = [p for p in sys.path if os.path.abspath(p) != script_dir]
+        import build
+    except ImportError:
+        print("📦 Installing build module...")
+        subprocess.run([sys.executable, "-m", "pip", "install", "build"], check=True)
+    finally:
+        sys.path = original_path
+
+    # Create output directory
+    print(f"📂 Preparing output directory: {output_dir}")
+    if os.path.exists(output_dir):
+        shutil.rmtree(output_dir)
+    os.makedirs(output_dir, exist_ok=True)
+
+    print("🏗️  Building PyPI package...")
+    print("   Using temporary directory to avoid module conflicts...")
+
+    # Run from temp directory to avoid local build.py shadowing the build module
+    with tempfile.TemporaryDirectory() as tmpdir:
+        print(f"   Temp directory: {tmpdir}")
+        print("   Running: python -m build -o <output_dir> <source_dir>")
+
+        result = subprocess.run(
+            [sys.executable, "-m", "build", "-o", output_dir, script_dir],
+            check=False,
+            cwd=tmpdir,
+        )
+
+    print("\n" + "=" * 40)
+    if result.returncode == 0:
+        print("✅ Build successful!")
+        print(f"📦 Files created in {output_dir}:")
+
+        files = os.listdir(output_dir)
+        if files:
+            for f in files:
+                file_path = os.path.join(output_dir, f)
+                size = os.path.getsize(file_path)
+                print(f"   📄 {f} ({size:,} bytes)")
+        else:
+            print("   (No files found)")
+
+        print("\n🚀 Ready for upload with:\n")
+        print("   - To test on Test PyPI:")
+        print(f"       python -m twine upload --repository testpypi {output_dir}/*")
+        print("\n   - To upload to PyPI (when ready):")
+        print(f"       python -m twine upload {output_dir}/*")
+    else:
+        print("❌ Build failed!")
+        print(f"   Exit code: {result.returncode}")
+        sys.exit(result.returncode)
+
+
+if __name__ == "__main__":
+    main()