refactor: overhaul armature validation system to be opt-in by default

- Change default validation mode from STRICT to NONE (disabled)
- Move validation from automatic panel draw to explicit "Validate Now" button
- Hide validation results when mode is changed to NONE
- Fix PMX/MMD model detection to check mmd_type value, not just attribute existence
- Add new validation result collapsible sections
- Improve UI presentation with better visual hierarchy
- Add translation strings for new validation UI elements

core/addon_preferences.py

@@ -63,6 +63,6 @@ def get_addon_preferences(context):
 # Initialize preferences if the file doesn't exist
 if not os.path.exists(PREFERENCES_FILE):
     save_preference("language", 0)  # Set default language to 0 (auto)
-    save_preference("validation_mode", "STRICT")  # Set default validation mode
+    save_preference("validation_mode", "NONE")  # Set default validation mode to NONE (off by default)
     save_preference("enable_logging", False) # Set default logging mode
     save_preference("highlight_problem_bones", True) # Set default bone highlighting