feat(series): add AJAX library switching and stacked-cascade CSS

Create web/src/series.ts with Alpine component implementing the same
switchLibrary pattern as the dashboard:
- Fetch /api/series on library change instead of full page reload
- Fade out/in transition with loading spinner
- Re-render series grid and pagination from JSON response
- Save selected library to localStorage

Import series.ts in main.ts.

Add stacked-cascade CSS to input.css for multi-cover series cards:
- Covers cascade from top-left to bottom-right with increasing z-index
- Front cover sits at bottom-right (highest z-index)
- Separate layout rules for 1-7 covers with rotation offsets
- Hover lift effect on series cards

web/static/input.css

@@ -398,4 +398,172 @@
     border-right: none;
     border-left: 1px solid var(--border);
   }
+
+  .stacked-covers {
+    position: relative;
+    width: 100%;
+    aspect-ratio: 2 / 3;
+    overflow: hidden;
+  }
+
+  .stacked-covers .cover-img {
+    position: absolute;
+    width: 70%;
+    height: 90%;
+    object-fit: cover;
+    border-radius: 4px;
+    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.4);
+  }
+
+  .stacked-covers.cover-count-1 .cover-0 {
+    top: 5%;
+    left: 15%;
+  }
+
+  .stacked-covers.cover-count-2 .cover-0 {
+    top: 8%;
+    left: 5%;
+    transform: rotate(-6deg);
+    z-index: 1;
+  }
+  .stacked-covers.cover-count-2 .cover-1 {
+    top: 2%;
+    left: 25%;
+    transform: rotate(3deg);
+    z-index: 2;
+  }
+
+  .stacked-covers.cover-count-3 .cover-0 {
+    top: 12%;
+    left: 2%;
+    transform: rotate(-8deg);
+    z-index: 1;
+  }
+  .stacked-covers.cover-count-3 .cover-1 {
+    top: 6%;
+    left: 15%;
+    transform: rotate(-2deg);
+    z-index: 2;
+  }
+  .stacked-covers.cover-count-3 .cover-2 {
+    top: 0;
+    left: 30%;
+    transform: rotate(4deg);
+    z-index: 3;
+  }
+
+  .stacked-covers.cover-count-4 .cover-0 {
+    top: 14%;
+    left: 0;
+    width: 65%;
+    height: 85%;
+    transform: rotate(-10deg);
+    z-index: 1;
+  }
+  .stacked-covers.cover-count-4 .cover-1 {
+    top: 10%;
+    left: 12%;
+    width: 65%;
+    height: 85%;
+    transform: rotate(-4deg);
+    z-index: 2;
+  }
+  .stacked-covers.cover-count-4 .cover-2 {
+    top: 5%;
+    left: 22%;
+    width: 65%;
+    height: 85%;
+    transform: rotate(2deg);
+    z-index: 3;
+  }
+  .stacked-covers.cover-count-4 .cover-3 {
+    top: 0;
+    left: 32%;
+    width: 65%;
+    height: 85%;
+    transform: rotate(6deg);
+    z-index: 4;
+  }
+
+  .stacked-covers.cover-count-5 .cover-0,
+  .stacked-covers.cover-count-6 .cover-0,
+  .stacked-covers.cover-count-7 .cover-0 {
+    top: 16%;
+    left: -2%;
+    width: 62%;
+    height: 82%;
+    transform: rotate(-12deg);
+    z-index: 1;
+  }
+
+  .stacked-covers.cover-count-5 .cover-1,
+  .stacked-covers.cover-count-6 .cover-1,
+  .stacked-covers.cover-count-7 .cover-1 {
+    top: 12%;
+    left: 8%;
+    width: 62%;
+    height: 82%;
+    transform: rotate(-6deg);
+    z-index: 2;
+  }
+
+  .stacked-covers.cover-count-5 .cover-2,
+  .stacked-covers.cover-count-6 .cover-2,
+  .stacked-covers.cover-count-7 .cover-2 {
+    top: 8%;
+    left: 17%;
+    width: 62%;
+    height: 82%;
+    transform: rotate(-1deg);
+    z-index: 3;
+  }
+
+  .stacked-covers.cover-count-5 .cover-3,
+  .stacked-covers.cover-count-6 .cover-3,
+  .stacked-covers.cover-count-7 .cover-3 {
+    top: 4%;
+    left: 26%;
+    width: 62%;
+    height: 82%;
+    transform: rotate(4deg);
+    z-index: 4;
+  }
+
+  .stacked-covers.cover-count-5 .cover-4,
+  .stacked-covers.cover-count-6 .cover-4,
+  .stacked-covers.cover-count-7 .cover-4 {
+    top: 0;
+    left: 35%;
+    width: 62%;
+    height: 82%;
+    transform: rotate(9deg);
+    z-index: 5;
+  }
+
+  .stacked-covers.cover-count-6 .cover-5,
+  .stacked-covers.cover-count-7 .cover-5 {
+    top: -2%;
+    left: 30%;
+    width: 60%;
+    height: 80%;
+    transform: rotate(13deg);
+    z-index: 6;
+  }
+
+  .stacked-covers.cover-count-7 .cover-6 {
+    top: -4%;
+    left: 25%;
+    width: 58%;
+    height: 78%;
+    transform: rotate(16deg);
+    z-index: 7;
+  }
+
+  .series-card {
+    transition: transform 0.2s ease, box-shadow 0.2s ease;
+  }
+
+  .series-card:hover {
+    transform: translateY(-4px);
+  }
 }