feat(web): add Discord link to sidebar account menu

Add a "Join our Discord" entry to the account dropdown's external-links
group, opening https://discord.gg/wdNEHETs87 in a new tab. lucide-react
has no Discord brand glyph, so include a small inline Discord SVG icon
(brand color). Add the joinDiscord label to all 8 locales.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/i18n/locales/zh-Hans.ts

@@ -36,6 +36,7 @@ const zhHans = {
     helpDocs: '帮助文档',
     featureRequest: '需求建议',
     starOnGitHub: '在 GitHub 上 Star',
+    joinDiscord: '加入 Discord 社区',
     create: '创建',
     edit: '编辑',
     delete: '删除',