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

locales/en.json

@@ -291,7 +291,15 @@
             "blurNsfwContent": "Blur NSFW Content",
             "blurNsfwContentHelp": "Blur mature (NSFW) content preview images",
             "showOnlySfw": "Show Only SFW Results",
-            "showOnlySfwHelp": "Filter out all NSFW content when browsing and searching"
+            "showOnlySfwHelp": "Filter out all NSFW content when browsing and searching",
+            "matureBlurThreshold": "Mature Blur Threshold",
+            "matureBlurThresholdHelp": "Set which rating level starts blur filtering when NSFW blur is enabled.",
+            "matureBlurThresholdOptions": {
+                "pg13": "PG13 and above",
+                "r": "R and above (default)",
+                "x": "X and above",
+                "xxx": "XXX only"
+            }
         },
         "videoSettings": {
             "autoplayOnHover": "Autoplay Videos on Hover",
@@ -1758,4 +1766,4 @@
             "retry": "Retry"
         }
     }
-}
+}