Add seasonal themes for Birthday, Olympia, Space, Sports, and Underwater: implement CSS and JavaScript for visual effects and configurations

Jellyfin.Plugin.Seasonals/Web/birthday.css

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+.birthday-container {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100vw;
+    height: 100vh;
+    pointer-events: none;
+    z-index: 9999;
+    overflow: hidden;
+    contain: strict;
+}
+
+.birthday-cake {
+    position: absolute;
+    bottom: 2vh;
+    left: 50vw;
+    transform: translateX(-50%);
+    font-size: 8rem;
+    z-index: 50;
+    filter: drop-shadow(0 0 10px rgba(255,255,255,0.4));
+}
+
+.birthday-cake img {
+    height: 15vh;
+    width: auto;
+    object-fit: contain;
+    max-height: 150px;
+}
+
+.birthday-symbol {
+    position: absolute;
+    bottom: -10vh; /* balloons rise from bottom */
+    animation: birthday-rise linear infinite;
+    font-size: 3rem;
+    opacity: 0.95;
+    z-index: 40;
+}
+
+.birthday-symbol img {
+    width: 6vh;
+    height: auto;
+    max-width: 60px;
+    object-fit: contain;
+}
+
+.birthday-confetti {
+    position: absolute;
+    top: -5vh;
+    width: 10px;
+    height: 10px;
+    opacity: 0.9;
+    animation: birthday-confetti-fall linear infinite;
+    z-index: 30;
+    /* Mix of circles and squares by using CSS variables or random in JS. For simplicity, we make all slightly rounded rectangles */
+    border-radius: 2px;
+}
+
+@keyframes birthday-rise {
+    0% { 
+        transform: translateY(10vh) rotate(var(--start-rot, 0deg)); 
+        opacity: 0; 
+    }
+    10% { 
+        opacity: 1; 
+    }
+    90% {
+        opacity: 1;
+    }
+    100% { 
+        transform: translateY(-110vh) rotate(calc(var(--start-rot, 0deg) * -1)); 
+        opacity: 0; 
+    }
+}
+
+@keyframes birthday-confetti-fall {
+    0% {
+        transform: translateY(-5vh) rotateX(0deg) rotateY(0deg) rotateZ(0deg);
+        opacity: 0;
+    }
+    5% {
+        opacity: 1;
+    }
+    90% {
+        opacity: 1;
+    }
+    100% {
+        transform: translateY(105vh) rotateX(720deg) rotateY(360deg) rotateZ(180deg);
+        opacity: 0;
+    }
+}