Blender 5.0 compat + STB XML import + sequence/stamps improvements

- Fix Blender 5.0 API: active_sequence_strip → active_strip, bracket
  property access on AddonPreferences, _RestrictContext during register()
- Fix new_effect() frame_end → length for Blender 5.0
- Fix OTIO adapter kwargs (rate/ignore_timecode_mismatch only for cmx_3600)
- Fix OTIO global_start_time RationalTime → int conversion
- Add Import STB XML operator with movie strip import and XML patching
  for Storyboard Pro transitions missing <alignment>
- Add Create Sequence Strip operator (select shots → create sequence)
- Improve Set Stamps: channel at top, no conflict with existing strips,
  use raw strip names in templates
- TVSHOW episode fixes: sequence loading, episode creation
- Kitsu: new_asset, new_episode, admin_connect fix, gazu version pin
- Fix escape warnings in file_utils regex patterns
- Guard handler registration against duplicates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/preferences.py

@@ -91,7 +91,11 @@ def load_prefs():
                 continue
             setattr(tracker_pref, k, os.path.expandvars(v))
 
-    prefs['tracker_name'] = prefs_datas['tracker_name']
+    try:
+        settings = get_scene_settings()
+        settings.tracker_name = prefs_datas['tracker_name']
+    except AttributeError:
+        pass  # Scene not available yet during register()
 
 
 class Trackers(PropertyGroup):
@@ -155,7 +159,7 @@ def register():
         
     config_path = os.getenv('VSE_TOOLBOX_CONFIG')
     if config_path:
-        prefs['config_path'] = os.path.expandvars(config_path)
+        prefs.config_path = os.path.expandvars(config_path)
 
     load_prefs()