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/utils/constants.matureBlurThreshold.test.js

@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+
+import { NSFW_LEVELS, getMatureBlurThreshold } from '../../../static/js/utils/constants.js';
+
+describe('getMatureBlurThreshold', () => {
+  it('returns configured PG13 threshold', () => {
+    expect(getMatureBlurThreshold({ mature_blur_level: 'PG13' })).toBe(NSFW_LEVELS.PG13);
+  });
+
+  it('normalizes lowercase values', () => {
+    expect(getMatureBlurThreshold({ mature_blur_level: 'x' })).toBe(NSFW_LEVELS.X);
+  });
+
+  it('falls back to R when value is invalid or missing', () => {
+    expect(getMatureBlurThreshold({ mature_blur_level: 'invalid' })).toBe(NSFW_LEVELS.R);
+    expect(getMatureBlurThreshold({})).toBe(NSFW_LEVELS.R);
+  });
+});