feat(ui): split book-card play action into reader/detail routing

The play button on book cards now opens the reader directly, instead of
always going to the detail page. Cards with an active progress sync
conflict route the play button to the detail page (which hosts the
conflict dialogue and resolves before writing progress), so the user is
never silently dropped into the reader with an unresolved conflict.

Backend:
- Add HasConflict to BookInfo and stamp it via ListSyncConflictsByUser
  (MarkActiveConflicts / MarkActiveConflictsSections) on the dashboard,
  bookshelf, series, tag, and search result card builders.
- Each page issues a single conflict query regardless of card count.

BookCard:
- Restructure into a detail link (cover + meta) with the play action as a
  sibling overlay using a pointer-events split: the container passes
  clicks through to detail while only the circular button routes to the
  reader. No nested anchors.
- On touch devices (hover: none) the play button stays visible.

Fix: carousel nav buttons had opacity-0 without pointer-events-none, so
they swallowed hover/clicks over book cards on the dashboard. They are
now click-through until the carousel is hovered.

web/static/input.css

@@ -412,6 +412,7 @@
 
   /* ---------- Book card (the atomic unit) ---------- */
   .book-card {
+    position: relative;
     background-color: var(--bg-secondary);
     border: 1px solid color-mix(in srgb, var(--text-primary) 6%, transparent);
     border-radius: 0.75rem;
@@ -442,14 +443,20 @@
   }
   .book-card-action {
     position: absolute;
-    inset: 0;
+    top: 0;
+    left: 0;
+    right: 0;
+    aspect-ratio: 2 / 3;
     display: flex;
     align-items: center;
     justify-content: center;
     opacity: 0;
+    pointer-events: none;
+    z-index: 2;
     transition: opacity 0.18s ease;
   }
-  .book-card:hover .book-card-action {
+  .book-card:hover .book-card-action,
+  .book-card:focus-within .book-card-action {
     opacity: 1;
   }
   .book-card-action-btn {
@@ -462,11 +469,21 @@
     background-color: color-mix(in srgb, var(--accent) 92%, white);
     color: #fff;
     box-shadow: 0 6px 18px rgba(0, 0, 0, 0.4);
+    pointer-events: auto;
+    text-decoration: none;
   }
   .book-card-meta {
     background-color: var(--bg-secondary);
     padding: 0.5rem 0.625rem;
   }
+  /* On touch devices there is no hover, so the quick-action stays visible and
+     only the circular button itself is clickable (the container passes clicks
+     through to the detail link). */
+  @media (hover: none) {
+    .book-card-action {
+      opacity: 1;
+    }
+  }
 
   /* ---------- Bold accents ---------- */