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"

frontend/src/hooks/usePhotosQuery.ts

@@ -5,11 +5,8 @@ import { usePhotosBackgroundLoadingStore } from '../store/photosBackgroundLoadin
 import api from '../services/api'
 import type { Photo } from '../types/photo'
 
-/** Reactive selector for the "more pages still streaming" flag. Used by
- *  thumbnail views to render a bottom-of-list spinner while
- *  usePhotosQuery's background cursor loop is still pulling pages. */
 export function usePhotosLoadingMore(): boolean {
-  return usePhotosBackgroundLoadingStore((s) => s.isLoadingMore)
+  return usePhotosBackgroundLoadingStore((s) => s.inFlight > 0)
 }
 
 /**
@@ -108,12 +105,10 @@ export function usePhotosQuery() {
 
       if (nextCursor) {
         // Fire-and-forget background loop using cursor chaining.
-        // The background store is reactive — Timeline et al. subscribe
-        // to render a "loading more…" indicator at the bottom of the
-        // grid while pages keep arriving. start/stop is balanced in a
-        // try/finally so an aborted/erroring loop can't leak the flag.
-        const bg = usePhotosBackgroundLoadingStore.getState()
-        bg.start()
+        // start/stop balanced in try/finally so an aborted or erroring
+        // loop can't leak the in-flight counter that drives the
+        // bottom-of-grid spinner.
+        usePhotosBackgroundLoadingStore.getState().start()
         void (async () => {
           try {
             for (let i = 0; i < MAX_PAGES && nextCursor; i++) {

frontend/src/store/photosBackgroundLoadingStore.ts

@@ -1,34 +1,17 @@
 import { create } from 'zustand'
 
-/**
- * Tracks whether usePhotosQuery's background cursor-chasing loop is
- * still pulling pages. Lives outside react-query because the loop runs
- * after the initial fetch resolves and react-query's isFetching only
- * covers fetches it owns. Consumers (Timeline, RatedView, ColorsView…)
- * use isLoadingMore to decide whether to render the "loading more
- * photos…" indicator at the bottom of the grid.
- */
+// Counter (not boolean) so an aborted-then-restarted loop on rapid
+// filter changes can't flip the flag false while a fresh loop is still
+// active. react-query's isFetching doesn't cover this loop because it
+// runs after queryFn resolves and writes via setQueryData directly.
 interface PhotosBackgroundLoadingState {
-  /** Count of in-flight background pagers. Stored as a counter so an
-   *  aborted-then-restarted loop on rapid filter changes can't flip the
-   *  flag false while a fresh loop is still active. */
   inFlight: number
-  isLoadingMore: boolean
   start: () => void
   stop: () => void
 }
 
 export const usePhotosBackgroundLoadingStore = create<PhotosBackgroundLoadingState>((set) => ({
   inFlight: 0,
-  isLoadingMore: false,
-  start: () =>
-    set((s) => ({
-      inFlight: s.inFlight + 1,
-      isLoadingMore: true,
-    })),
-  stop: () =>
-    set((s) => {
-      const next = Math.max(0, s.inFlight - 1)
-      return { inFlight: next, isLoadingMore: next > 0 }
-    }),
+  start: () => set((s) => ({ inFlight: s.inFlight + 1 })),
+  stop: () => set((s) => ({ inFlight: Math.max(0, s.inFlight - 1) })),
 }))