feat(header): add GitHub star button to top bar

Compact pill (GitHub icon | star count) placed left of the version
display. Star count is fetched from the GitHub API once per 24h and
cached in localStorage; the button hides entirely when no count is
available (e.g. air-gapped deployments), matching the existing update-
check graceful-degradation pattern. Fetch is bounded by a 4s abort
timeout to avoid hanging requests offline.

src/locales/ja-JP/common.ts

@@ -170,6 +170,7 @@ export default {
   'common.time.hour': '時間',
   'common.time.minute': '分',
   'common.issue.report': '問題を報告',
+  'common.github.star.tooltip': 'GitHub でスターをつける',
   'common.social.discord': 'Discordに参加',
   'common.table.mark': 'コメント',
   'common.table.rollback.mark': 'ロールバックコメント',