Add Kindle EPUB compatibility fixer

Introduce an optional Kindle EPUB compatibility fixer and integrate it into the ebook organization flow. Adds a new config key (ebook_kindle_fix_enabled, default false), a settings API update, and a UI toggle (visible when preferred format is EPUB). Implements src/lib/utils/epub-fixer.ts (uses adm-zip and cheerio) to apply fixes: add UTF-8 XML declarations, remove body/#bodymatter fragments from links, validate/normalize dc:language, and remove stray <img> tags without src. organize-files.processor now detects EPUB downloads, runs the fixer (produces a temp fixed EPUB), uses the fixed file for organization, logs fixes, and cleans up temporary files; fix failures are non-blocking and the original download is preserved. Adds dependencies adm-zip and @types/adm-zip and updates documentation and types/UI to expose the new setting. Also includes helper functions to detect EPUB paths in downloads.

src/app/api/admin/settings/route.ts

@@ -141,6 +141,8 @@ export async function GET(request: NextRequest) {
         preferredFormat: configMap.get('ebook_sidecar_preferred_format') || 'epub',
         // Auto-grab: default true to preserve existing behavior
         autoGrabEnabled: configMap.get('ebook_auto_grab_enabled') !== 'false',
+        // Kindle compatibility fixes: default false
+        kindleFixEnabled: configMap.get('ebook_kindle_fix_enabled') === 'true',
       },
       general: {
         appName: configMap.get('app_name') || 'ReadMeABook',