fix: server-side comic page extraction with caching

Fixes #3505 - Large comic books (300+ MB) were unusable due to
client-side extraction.

Changes:
- Add ComicCacheManager for server-side page extraction and caching
- Add GET /api/items/:id/comic-pages endpoint for page metadata
- Add GET /api/items/:id/comic-page/:page endpoint for individual pages
- Update ComicReader.vue to fetch pages on-demand from server
- Add browser-side preloading for adjacent pages

Before: Client downloads entire comic file, extracts in browser
After: Server extracts pages on-demand, caches to disk, streams to client

Performance improvements:
- Initial load: Only metadata request (~1KB) instead of full file (300MB+)
- Page turns: Single image request (~100KB-2MB) with disk caching
- Memory: No longer loads entire archive in browser memory
- Subsequent views: Cached pages served instantly from disk

server/routers/ApiRouter.js

@@ -126,6 +126,9 @@ class ApiRouter {
     this.router.get('/items/:id/file/:fileid/download', LibraryItemController.middleware.bind(this), LibraryItemController.downloadLibraryFile.bind(this))
     this.router.get('/items/:id/ebook/:fileid?', LibraryItemController.middleware.bind(this), LibraryItemController.getEBookFile.bind(this))
     this.router.patch('/items/:id/ebook/:fileid/status', LibraryItemController.middleware.bind(this), LibraryItemController.updateEbookFileStatus.bind(this))
+    // Comic page routes - server-side extraction with caching for performance
+    this.router.get('/items/:id/comic-pages/:fileid?', LibraryItemController.middleware.bind(this), LibraryItemController.getComicPages.bind(this))
+    this.router.get('/items/:id/comic-page/:page/:fileid?', LibraryItemController.middleware.bind(this), LibraryItemController.getComicPage.bind(this))
 
     //
     // User Routes