fix(reading-position): fix infinite loop causing spam saves

The root cause was scheduleSave being in the scroll effect's dependency array.
Even though scheduleSave had an empty dependency array, React still saw it as
a dependency and re-ran the effect constantly, causing unmount/remount loops
and triggering flush-on-unmount repeatedly.

Solution: Store scheduleSave in a ref (scheduleSaveRef) and call it via the ref
in the scroll handler. This removes scheduleSave from the effect dependencies
while still allowing the scroll handler to access the latest version.

This fixes the "Maximum update depth exceeded" error and stops the spam saves.

src/hooks/useReadingPosition.ts

@@ -31,6 +31,7 @@ export const useReadingPosition = ({
   const suppressUntilRef = useRef<number>(0)
   const syncEnabledRef = useRef(syncEnabled)
   const onSaveRef = useRef(onSave)
+  const scheduleSaveRef = useRef<((pos: number) => void) | null>(null)
 
   // Keep refs in sync with props
   useEffect(() => {
@@ -91,6 +92,11 @@ export const useReadingPosition = ({
     }, DEBOUNCE_MS)
   }, [])
 
+  // Store scheduleSave in ref for use in scroll handler
+  useEffect(() => {
+    scheduleSaveRef.current = scheduleSave
+  }, [scheduleSave])
+
   // Immediate save function
   const saveNow = useCallback(() => {
     if (!syncEnabledRef.current || !onSaveRef.current) return
@@ -143,7 +149,7 @@ export const useReadingPosition = ({
 
       // Schedule auto-save if sync is enabled (unless suppressed)
       if (Date.now() >= suppressUntilRef.current) {
-        scheduleSave(clampedProgress)
+        scheduleSaveRef.current?.(clampedProgress)
       } else {
         const remainingMs = suppressUntilRef.current - Date.now()
         console.log(`[reading-position] [${new Date().toISOString()}] 🛡️ Save suppressed (${remainingMs}ms remaining) at ${Math.round(clampedProgress * 100)}%`)
@@ -207,7 +213,7 @@ export const useReadingPosition = ({
         clearTimeout(completionTimerRef.current)
       }
     }
-  }, [enabled, onPositionChange, onReadingComplete, readingCompleteThreshold, scheduleSave, completionHoldMs])
+  }, [enabled, onPositionChange, onReadingComplete, readingCompleteThreshold, completionHoldMs])
 
   // Reset reading complete state when enabled changes
   useEffect(() => {