feat(#4241): add Vietnamese (vi) as 14th UI locale + wire parity

Maintainer-completed @thanhtantran's #4241 (Nathan approved bringing it in):
- vi locale block in static/i18n.js (contributor's 1615-line translation) rebased onto master.
- Added vi to _LOGIN_LOCALE in api/routes.py (login-page localization, issue #1442 parity).
- Bumped the per-locale hardcoded-count assertions 13->14 across the locale-parity
  tests (rtl, quota_chip, hide_suggestions, virtualize_transcript, no_git, help_tab,
  verdigris/zeus skins, auxiliary_models) and added 'vi' to the composer-voice-buttons
  LOCALES tuples. vi already carries all current English keys (verified 14x per key).
All 497 locale/parity tests pass. Co-authored-by: thanhtantran

tests/test_pr1721_rtl_salvage.py

@@ -112,5 +112,5 @@ def test_rtl_in_config_defaults_and_writable_keys():
 def test_rtl_localized_in_all_locales():
     js = I18N.read_text(encoding="utf-8")
     # Count occurrences — should match the 11 locale blocks
-    assert js.count("settings_label_rtl:") == 13
-    assert js.count("settings_desc_rtl:") == 13
+    assert js.count("settings_label_rtl:") == 14
+    assert js.count("settings_desc_rtl:") == 14