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/locales/en-US.ts

@@ -1337,6 +1337,7 @@ export default {
       reloading: 'Applying configuration...',
       qbittorrent: 'Qbittorrent',
       transmission: 'Transmission',
+      rtorrent: 'rTorrent',
       emby: 'Emby',
       jellyfin: 'Jellyfin',
       plex: 'Plex',
@@ -2782,6 +2783,7 @@ export default {
     type: 'Type',
     enabled: 'Enabled',
     customTypeHint: 'Custom downloader type, for plugin scenarios',
+    rtorrentHostHint: 'HTTP: http://ip:port/RPC2 or SCGI: scgi://ip:port',
     default: 'Default',
     host: 'Host',
     username: 'Username',

src/locales/zh-CN.ts

@@ -1332,6 +1332,7 @@ export default {
       reloading: '正在应用配置...',
       qbittorrent: 'Qbittorrent',
       transmission: 'Transmission',
+      rtorrent: 'rTorrent',
       emby: 'Emby',
       jellyfin: 'Jellyfin',
       plex: 'Plex',
@@ -2746,6 +2747,7 @@ export default {
     type: '类型',
     enabled: '启用',
     customTypeHint: '自定义下载器类型，用于插件等场景',
+    rtorrentHostHint: 'HTTP: http://ip:port/RPC2 或 SCGI: scgi://ip:port',
     default: '默认',
     host: '地址',
     username: '用户名',

src/locales/zh-TW.ts

@@ -1333,6 +1333,7 @@ export default {
       reloading: '正在應用配置...',
       qbittorrent: 'Qbittorrent',
       transmission: 'Transmission',
+      rtorrent: 'rTorrent',
       emby: 'Emby',
       jellyfin: 'Jellyfin',
       plex: 'Plex',
@@ -2746,6 +2747,7 @@ export default {
     name: '名稱',
     type: '類型',
     customTypeHint: '自定義下載器類型，用於插件等場景',
+    rtorrentHostHint: 'HTTP: http://ip:port/RPC2 或 SCGI: scgi://ip:port',
     enabled: '啟用',
     default: '預設',
     host: '地址',