Add seasonal effects for Pi Day, Pride, Rain, and Storm; enhance existing styles

- Introduced new CSS and JS files for Pi Day, Pride, Rain, and Storm effects.
- Updated existing seasonal styles (e.g., Halloween, Hearts, Resurrection) to improve performance with 'contain: layout paint'.
- Enhanced animations for seasonal effects, including adjustments to keyframes and element creation logic.
- Added configuration options for new effects in the main seasonals.js file.
- Updated test-site.html to include new seasonal options in the dropdown.

Jellyfin.Plugin.Seasonals/Web/rain.css

@@ -0,0 +1,26 @@
+.rain-container {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100vw;
+    height: 100vh;
+    pointer-events: none;
+    z-index: 1000;
+    overflow: hidden;
+    contain: layout paint;
+}
+
+.raindrop-pure {
+    position: absolute;
+    width: 2px;
+    height: 40px;
+    background: linear-gradient(to bottom, rgba(200, 200, 255, 0), rgba(200, 200, 255, 0.7));
+    transform-origin: bottom;
+}
+
+@keyframes pure-rain {
+    0% { transform: translateY(-30vh) translateX(0) rotate(20deg); opacity: 0; }
+    5% { opacity: 1; }
+    95% { opacity: 1; }
+    100% { transform: translateY(110vh) translateX(-40vh) rotate(20deg); opacity: 0; }
+}