refactor: simplify after self-review

- drop the redundant isLoadingMore boolean from the background-loading
  store; derive from inFlight > 0 in the selector
- pull _active_source_root_paths out of get_library_stats and reuse it
  from get_duplicate_groups (same is_active + admin-scope check, now
  in one place)
- drop p.rstrip('/') in the duplicates folder-scope filter (SourceRoot
  paths are never written with a trailing slash)
- match Timeline's initial-load affordance to the new bottom indicator
  (Loader2 spinner + ellipsis instead of plain "Loading photos...")
- trim narrative comments that explained what the surrounding code does

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/timeline/Timeline.tsx

@@ -199,10 +199,6 @@ export function Timeline() {
   // Shared photos query — both Timeline and PreviewView use the same hook so
   // they share one cache entry, regardless of filter state.
   const { data: photos = [], isLoading } = usePhotosQuery()
-  // Background cursor loop (after the initial page resolves) still
-  // streaming more photos? Drives the bottom-of-grid spinner so the
-  // user has a signal that more results are on the way and a sudden
-  // "end of list" is real rather than mid-fetch.
   const isLoadingMore = usePhotosLoadingMore()
 
   // Tracks the previous viewMode so the "preview just closed" scroll
@@ -736,8 +732,9 @@ export function Timeline() {
 
   if (isLoading) {
     return (
-      <div className="flex items-center justify-center h-full">
-        <div className="text-text-muted">Loading photos...</div>
+      <div className="flex h-full items-center justify-center gap-2 text-text-muted">
+        <Loader2 className="h-4 w-4 animate-spin" />
+        Loading photos…
       </div>
     )
   }
@@ -794,11 +791,7 @@ export function Timeline() {
             position: 'relative',
           }}
         >
-          {/* Bottom loading indicator. Positioned absolutely just below
-           *  the virtualizer's last row so a mid-grid spinner doesn't
-           *  jitter the layout when pages flip. Sits inside the same
-           *  positioned wrapper that hosts the virtual items so the
-           *  parent's translate space stays self-contained. */}
+          {/* Absolute-positioned so layout doesn't jitter when pages flip. */}
           {isLoadingMore && photos.length > 0 && (
             <div
               className="pointer-events-none absolute left-0 right-0 flex items-center justify-center gap-2 py-4 text-xs text-text-muted"