feat: add configurable mature blur threshold setting

Add new setting 'mature_blur_level' with options PG13/R/X/XXX to control
which NSFW rating level triggers blur filtering when NSFW blur is enabled.

- Backend: update preview selection logic to respect threshold
- Frontend: update UI components to use configurable threshold
- Settings: add validation and normalization for mature_blur_level
- Tests: add coverage for new threshold behavior
- Translations: add keys for all supported languages

Fixes #867

tests/frontend/state/index.test.js

@@ -15,7 +15,8 @@ describe('state module', () => {
     expect(defaultSettings).toMatchObject({
       civitai_api_key: '',
       language: 'en',
-      blur_mature_content: true
+      blur_mature_content: true,
+      mature_blur_level: 'R'
     });
 
     expect(defaultSettings.download_path_templates).toEqual(DEFAULT_PATH_TEMPLATES);