fix(webp): address review — drop broken GIF, robust ext check, centralize lists

Review feedback on #1831:
- Remove *.gif from the save/output dialog filter (PR had added it there).
  Verified empirically that cv2.imread/imwrite cannot decode OR encode GIF on
  OpenCV 4.10 *or* 4.11 (write raises, read returns None), so GIF silently
  failed on both ends — dropped from every dialog and from has_image_extension.
- has_image_extension now uses os.path.splitext so only the true extension
  counts ('photo.png.bak' / 'clip.webp.mp4' are no longer treated as images).
- Centralize the supported-extension set in modules.globals (IMAGE_EXTENSIONS /
  VIDEO_EXTENSIONS); file_types, all QFileDialog filters and has_image_extension
  now derive from it instead of hand-copied lists that had already drifted.

WEBP itself is unchanged and works (libwebp ships with opencv-python).

modules/ui.py

@@ -236,6 +236,15 @@ _RECENT_SOURCE_DIR: Optional[str] = None
 _RECENT_TARGET_DIR: Optional[str] = None
 _RECENT_OUTPUT_DIR: Optional[str] = None
 
+# QFileDialog filter strings, built from the canonical extension sets in
+# globals so every dialog stays in sync (no hand-copied lists to drift).
+_IMAGE_FILE_FILTER = "Images (" + " ".join(
+    f"*{ext}" for ext in modules.globals.IMAGE_EXTENSIONS
+) + ")"
+_MEDIA_FILE_FILTER = "Media (" + " ".join(
+    f"*{ext}" for ext in (*modules.globals.IMAGE_EXTENSIONS, *modules.globals.VIDEO_EXTENSIONS)
+) + ")"
+
 
 # ─── image utilities ─────────────────────────────────────────────────────
 
@@ -733,7 +742,7 @@ class MainWindow(QMainWindow):
         path, _filter = QFileDialog.getOpenFileName(
             self, _("select an source image"),
             _RECENT_SOURCE_DIR or "",
-            "Images (*.png *.jpg *.jpeg *.gif *.bmp *.webp)",
+            _IMAGE_FILE_FILTER,
         )
         if path and is_image(path):
             modules.globals.source_path = path
@@ -754,7 +763,7 @@ class MainWindow(QMainWindow):
         path, _filter = QFileDialog.getOpenFileName(
             self, _("select an target image or video"),
             _RECENT_TARGET_DIR or "",
-            "Media (*.png *.jpg *.jpeg *.gif *.bmp *.webp *.mp4 *.mkv)",
+            _MEDIA_FILE_FILTER,
         )
         if not path:
             return
@@ -885,7 +894,7 @@ class MainWindow(QMainWindow):
             path, _f = QFileDialog.getSaveFileName(
                 self, _("save image output file"),
                 os.path.join(_RECENT_OUTPUT_DIR or "", "output.png"),
-                "Images (*.png *.jpg *.jpeg *.gif *.bmp *.webp)",
+                _IMAGE_FILE_FILTER,
             )
         elif is_video(modules.globals.target_path):
             path, _f = QFileDialog.getSaveFileName(
@@ -1333,7 +1342,7 @@ class MapperDialog(QDialog):
         path, _f = QFileDialog.getOpenFileName(
             self, _("select an source image"),
             _RECENT_SOURCE_DIR or "",
-            "Images (*.png *.jpg *.jpeg *.gif *.bmp *.webp)",
+            _IMAGE_FILE_FILTER,
         )
         if not path:
             return
@@ -1438,7 +1447,7 @@ class LiveMapperDialog(QDialog):
         path, _f = QFileDialog.getOpenFileName(
             self, _("select an source image"),
             _RECENT_SOURCE_DIR or "",
-            "Images (*.png *.jpg *.jpeg *.gif *.bmp *.webp)",
+            _IMAGE_FILE_FILTER,
         )
         if not path:
             return