feat(backgrounds): add matrix-rain background

src/locales/en.json

@@ -1159,6 +1159,8 @@
       "snowfallDesc": "Falling snow with wind drift",
       "starfield": "Starfield",
       "starfieldDesc": "Stars flying toward the viewer",
+      "matrixRain": "Matrix Rain",
+      "matrixRainDesc": "Falling columns of glyphs",
       "geminiEffect": "Gemini Effect",
       "geminiEffectDesc": "SVG effect like Google Gemini",
       "speed": "Speed",
@@ -1181,6 +1183,7 @@
       "multicolor": "Multicolor",
       "lineColor": "Line Color",
       "wind": "Wind",
+      "charset": "Charset",
       "bgColor": "Background Color",
       "fillColor": "Fill Color",
       "color1": "Color 1",