fix(dashboard): stop duplicating items after scan completion

The scan-complete handler in dashboard.ts attempted to deduplicate book
cards by querying [data-media-item-id], but neither the client-side
renderBookCard nor the server-side BookCard template ever set that
attribute. As a result the dedup Set was always empty, every item from
the API response was treated as new, and all items were prepended via
insertAdjacentHTML('afterbegin', ...) on every 5-minute scan — causing
visible duplication (doubling, tripling) that only cleared on page
refresh.

Fix by replacing the fragile dedup-and-prepend logic with a per-track
full innerHTML replace. This is simpler, correctly handles items that
should be removed after a scan (the old code never removed anything),
and also removes stale sections no longer returned by the API.

Additional hardening:
- Add data-media-item-id to both renderBookCard (dashboard.ts) and the
  server-side card wrapper (dashboard.templ) so server-rendered and
  JS-rendered cards are structurally identical.
- Guard the bookhoard:scan-complete listener registration with a
  module-level boolean (scanListenerRegistered) so the handler cannot
  accumulate if Alpine ever re-inits the body subtree.
- Remove debug console.log statements from the scan handler.

templates/dashboard.templ

@@ -77,11 +77,11 @@ templ CollectionCarousel(section handlers.SectionData) {
 				class="carousel-track flex gap-4 overflow-x-auto scroll-smooth snap-x snap-mandatory px-6 pb-2"
 				style="scrollbar-width: none; -ms-overflow-style: none;"
 			>
-				for _, item := range section.Items {
-					<div class="flex-shrink-0 w-36 sm:w-40 snap-start">
-						@BookCard(item)
-					</div>
-				}
+			for _, item := range section.Items {
+				<div class="flex-shrink-0 w-36 sm:w-40 snap-start" data-media-item-id={ item.MediaItemID }>
+					@BookCard(item)
+				</div>
+			}
 				if len(section.Items) == 0 {
 					<div class="flex flex-col items-center justify-center text-center py-12 w-full gap-2" style="color: var(--text-secondary);">
 						@Icon("book", "h-8 w-8 opacity-50")