feat: Add Windows compatibility, PyInstaller build setup, and custom icon

- Add Windows LibreOffice detection fallback for soffice path
  - Check standard install locations (%PROGRAMFILES%, %PROGRAMFILES(X86)%)
  - Graceful fallback with clear error messages

- Add PyInstaller build infrastructure
  - build/ccma.spec: PyInstaller configuration with icon generation
  - build/build.ps1: Automated build script for standalone exe
  - main.py: Entry point for PyInstaller
  - Supports versioning and architecture tagging

- Create custom CCMA icon
  - Person + Gear symbol representing member administration
  - Cyan/White/Orange color scheme
  - Auto-converts PNG to ICO during build

- Update documentation
  - README: Windows PowerShell setup instructions
  - README: Linux/macOS bash setup instructions
  - README: Standalone executable build guide
  - pyproject.toml: Add 'build' extra with pyinstaller+pillow

- Add regression tests
  - Test office executable detection and Windows fallback
  - Verify all tests pass (80 passed)

src/ccma/services/documents.py

@@ -474,7 +474,7 @@ def _slot_at(boundaries: list[tuple[int, int]], position: int) -> tuple[int, int
 
 
 def _convert_to_pdf(source: Path, output_directory: Path) -> Path:
-    executable = shutil.which("soffice") or shutil.which("libreoffice")
+    executable = _office_executable()
     if not executable:
         raise DocumentError("LibreOffice/OpenOffice wurde nicht gefunden; PDF-Erzeugung ist nicht möglich.")
     profile = output_directory / "libreoffice-profile"
@@ -499,6 +499,24 @@ def _convert_to_pdf(source: Path, output_directory: Path) -> Path:
     return converted
 
 
+def _office_executable() -> str | None:
+    executable = shutil.which("soffice") or shutil.which("libreoffice")
+    if executable:
+        return executable
+    if os.name != "nt":
+        return None
+    windows_roots: list[str] = []
+    for variable in ("PROGRAMFILES", "ProgramW6432", "PROGRAMFILES(X86)"):
+        value = os.environ.get(variable)
+        if value and value not in windows_roots:
+            windows_roots.append(value)
+    for root in windows_roots:
+        candidate = Path(root) / "LibreOffice" / "program" / "soffice.exe"
+        if candidate.is_file():
+            return str(candidate)
+    return None
+
+
 def _safe_pdf_name(value: str) -> str:
     stem = Path(value.strip()).stem
     stem = re.sub(r"[^A-Za-z0-9ÄÖÜäöüß._ -]+", "-", stem).strip(" .-")