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/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++) {