feat: add image caching for offline mode

- Add imageCacheService with localStorage-based image caching and LRU eviction
- Create useImageCache hook for React components to fetch and cache images
- Integrate image caching with article service to cache cover images on load
- Add image cache settings (enable/disable, size limit) to user settings
- Update ReaderHeader to use cached images for article covers
- Update BookmarkViews (CardView, LargeView) to use cached images
- Add image cache configuration UI in OfflineModeSettings with:
  - Toggle to enable/disable image caching
  - Slider to set cache size limit (10-200 MB)
  - Display current cache stats (size and image count)
  - Clear cache button

Images are cached in localStorage for offline viewing, with a configurable
size limit (default 50MB). LRU eviction ensures cache stays within limits.

src/components/ReaderHeader.tsx

@@ -2,6 +2,8 @@ import React from 'react'
 import { FontAwesomeIcon } from '@fortawesome/react-fontawesome'
 import { faHighlighter, faClock } from '@fortawesome/free-solid-svg-icons'
 import { format } from 'date-fns'
+import { useImageCache } from '../hooks/useImageCache'
+import { UserSettings } from '../services/settingsService'
 
 interface ReaderHeaderProps {
   title?: string
@@ -11,6 +13,7 @@ interface ReaderHeaderProps {
   readingTimeText?: string | null
   hasHighlights: boolean
   highlightCount: number
+  settings?: UserSettings
 }
 
 const ReaderHeader: React.FC<ReaderHeaderProps> = ({
@@ -20,13 +23,15 @@ const ReaderHeader: React.FC<ReaderHeaderProps> = ({
   published,
   readingTimeText,
   hasHighlights,
-  highlightCount
+  highlightCount,
+  settings
 }) => {
+  const cachedImage = useImageCache(image, settings)
   const formattedDate = published ? format(new Date(published * 1000), 'MMM d, yyyy') : null
-  if (image) {
+  if (cachedImage) {
     return (
       <div className="reader-hero-image">
-        <img src={image} alt={title || 'Article image'} />
+        <img src={cachedImage} alt={title || 'Article image'} />
         {formattedDate && (
           <div className="publish-date-topright">
             {formattedDate}