feat(downloader): add rTorrent UI support

Add rTorrent as a downloader option in settings, setup wizard, and
downloader card with config form (host, username, password) and
ruTorrent logo. Include i18n translations for zh-CN, zh-TW, and en-US.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/api/constants.ts

@@ -52,6 +52,10 @@ export const downloaderOptions = [
     value: 'transmission',
     title: i18n.global.t('setting.system.transmission'),
   },
+  {
+    value: 'rtorrent',
+    title: i18n.global.t('setting.system.rtorrent'),
+  },
 ]
 
 export const downloaderDict = downloaderOptions.reduce((dict, item) => {