refactor: hook dependencies name everything the hooks read

The 12 react-hooks/exhaustive-deps warnings the gate let through:
- ShareDialog and YearFilter built a fresh [] per render while their query loaded, so their
  memos recomputed on every render; they share one empty list now.
- VideoEditor's drag effect called a seek/moveBoundary recreated each render; both are stable
  callbacks in its deps.
- WizardProvider re-checks onboarding on exactly the fields it reads (not the me object, whose
  60 s refetch would reopen a closed wizard).
- Channels, Feed and PlexPlayer list the stable values they already used.
- The player's keydown effect is bound once on purpose (its helpers read refs): said so, with a
  disable, as the codebase's other deliberate sites do.
- dialogFocus and the YouTube player's cleanup read refs React may already have let go of; they
  use what they captured when the effect ran.

frontend/src/components/Feed.tsx

@@ -230,15 +230,15 @@ export default function Feed({
   // rows are unchanged, so clearing overrides there would make a just-hidden card flash back.
   // Only a real refetch (page count doesn't grow) is authoritative.
   const pagesLenRef = useRef(0);
+  const pagesLen = query.data?.pages?.length ?? 0;
   useEffect(() => {
-    const len = query.data?.pages?.length ?? 0;
-    const appended = len > pagesLenRef.current;
-    pagesLenRef.current = len;
+    const appended = pagesLen > pagesLenRef.current;
+    pagesLenRef.current = pagesLen;
     if (!appended) {
       setOverrides({});
       setSavedOverrides({});
     }
-  }, [query.dataUpdatedAt]);
+  }, [query.dataUpdatedAt, pagesLen]);
 
   // One shared query with the sidebar (same key + filters -> React Query dedupes to a single request):
   // the feed reads `.count`, the sidebar reads `.facets` (PERF-13, replaces feedCount + facets).