fix(reading-position): use throttle instead of debounce for saves

Changed from debounce (which resets timer on every scroll) to throttle
(which saves at regular 3s intervals). This ensures position is saved
during continuous slow scrolling.

Key changes:
- Don't reset timer if one is already pending
- Track latest position in pendingPositionRef
- Save the latest position when timer fires, not the position from when scheduled

This prevents the issue where slow continuous scrolling would never
trigger a save because the debounce timer kept resetting.

src/hooks/useReadingPosition.ts

@@ -29,6 +29,7 @@ export const useReadingPosition = ({
   const completionTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
   const lastSavedAtRef = useRef<number>(0)
   const suppressUntilRef = useRef<number>(0)
+  const pendingPositionRef = useRef<number>(0) // Track latest position for throttled save
 
   // Suppress auto-saves for a given duration (used after programmatic restore)
   const suppressSavesFor = useCallback((ms: number) => {
@@ -37,7 +38,7 @@ export const useReadingPosition = ({
     console.log(`[reading-position] [${new Date().toISOString()}] 🛡️ Suppressing saves for ${ms}ms until ${new Date(until).toISOString()}`)
   }, [])
 
-  // Debounced save function - simple 2s debounce
+  // Throttled save function - saves at 3s intervals, not debounced
   const scheduleSave = useCallback((currentPosition: number) => {
     if (!syncEnabled || !onSave) {
       return
@@ -65,20 +66,26 @@ export const useReadingPosition = ({
       return
     }
 
-    // Clear any existing timer and schedule new save
+    // Update the pending position (latest position to save)
+    pendingPositionRef.current = currentPosition
+
+    // Throttle: only schedule a save if one isn't already pending
+    // This ensures saves happen at regular 3s intervals during continuous scrolling
     if (saveTimerRef.current) {
-      clearTimeout(saveTimerRef.current)
+      return // Already have a save scheduled, don't reset the timer
     }
 
-    const DEBOUNCE_MS = 3000 // Save max every 3 seconds
+    const THROTTLE_MS = 3000 // Save every 3 seconds during scrolling
     saveTimerRef.current = setTimeout(() => {
-      console.log(`[reading-position] [${new Date().toISOString()}] 💾 Auto-save at ${Math.round(currentPosition * 100)}%`)
-      lastSavedPosition.current = currentPosition
+      // Save the latest position, not the one from when timer was scheduled
+      const positionToSave = pendingPositionRef.current
+      console.log(`[reading-position] [${new Date().toISOString()}] 💾 Auto-save at ${Math.round(positionToSave * 100)}%`)
+      lastSavedPosition.current = positionToSave
       hasSavedOnce.current = true
       lastSavedAtRef.current = Date.now()
-      onSave(currentPosition)
+      onSave(positionToSave)
       saveTimerRef.current = null
-    }, DEBOUNCE_MS)
+    }, THROTTLE_MS)
   }, [syncEnabled, onSave])
 
   // Immediate save function