Show a clear message when PyMuPDF is missing

src/pdf_runtime.py

@@ -0,0 +1,15 @@
+"""Small helpers for optional PDF runtime dependencies."""
+
+PDF_VIEWER_PYMUPDF_MISSING = (
+    "PDF viewer requires PyMuPDF. Install optional PDF dependencies with "
+    "`pip install -r requirements-optional.txt` (PyMuPDF is AGPL-3.0)."
+)
+
+
+def load_pymupdf_for_pdf_viewer():
+    """Return the PyMuPDF module, or raise a user-facing setup hint."""
+    try:
+        import fitz  # PyMuPDF, optional
+    except ImportError as exc:
+        raise RuntimeError(PDF_VIEWER_PYMUPDF_MISSING) from exc
+    return fitz