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/ja-JP.ts

@@ -38,6 +38,7 @@ const jaJP = {
     helpDocs: 'ヘルプドキュメント',
     featureRequest: '機能リクエスト',
     starOnGitHub: 'GitHubでStarする',
+    joinDiscord: 'Discord に参加',
     create: '作成',
     edit: '編集',
     delete: '削除',