Flatpak: bypass XDG portal for .tum open dialog

Native file dialog routes through the XDG document portal, which exposes
only the selected file at /run/user/UID/doc/... — siblings (param.yaml,
.py) are unreachable. Force Qt's non-native dialog in Flatpak so it walks
the real filesystem via --filesystem=home and returns a usable path.

src/testium/main_win/test_file_manager.py

@@ -212,8 +212,17 @@ class TestFileManager:
         d = ""
         if w.testFile is not None:
             d = os.path.dirname(w.testFile)
+        # In Flatpak the native dialog goes through the XDG document portal,
+        # which returns /run/user/UID/doc/.../test.tum and only exposes the
+        # selected file — sibling files (param.yaml, .py, etc.) are unreachable.
+        # Force Qt's own dialog, which walks the real filesystem mounted via
+        # --filesystem=home and returns a regular path with sibling access.
+        options = QFileDialog.Options()
+        if os.path.isfile("/.flatpak-info"):
+            options |= QFileDialog.Option.DontUseNativeDialog
         file_name, _ = QFileDialog.getOpenFileName(
-            w, "Open the test file", d, "testium file (*.tum);;All Files (*)"
+            w, "Open the test file", d,
+            "testium file (*.tum);;All Files (*)", options=options
         )
         if file_name:
             self.reload(file_name)