fix(i18n): add worktree translation to all 19 third-language dicts (#285)

PR #285 added the zh/en 'worktree' key but the 19 third-language dicts
(de/fr/pt/ko/ar/hi/id/tr/vi/th/ru/it/nl/sv/pl/zhHK/zhTW/zhMO) were missing
it — only ja was patched in the prior commit. The dicts are typed
Record<keyof typeof zh, string> via an unchecked 'as' cast, so tsc did
not catch the gap.

This commit:
- adds the worktree translation to all 19 dicts
- strengthens tests/locales.spec.ts to assert key-set equality for EVERY
  shipped dictionary (not just zh/en/ja), preventing future regressions

src/client/locales-it.ts

@@ -97,6 +97,7 @@ export const it: Record<string, string> = {
   commit: 'Commit',
   commitError: 'Commit non riuscito',
   branch: 'Ramo',
+  worktree: 'Albero di lavoro',
   checkoutError: 'Cambio di ramo non riuscito',
   history: 'Cronologia',
   changes: 'Modifiche',