feat(i18n): add Italian (it) locale

Adds complete Italian translation for all ~280 UI strings in static/i18n.js
and the login page strings in api/routes.py (_LOGIN_LOCALE).

Ordered alphabetically: en → it → ja in both files.
Preserves all JS function templates, template literals, and plural forms.

(cherry picked from commit c66e04b190)

api/routes.py

@@ -1944,6 +1944,15 @@ _LOGIN_LOCALE = {
     # Strings mirror static/i18n.js login_* keys for the corresponding locale.
     # See issue #1442. When adding a new locale to LOCALES in i18n.js, also add
     # the matching entry here — tests/test_login_locale_parity.py enforces this.
+    "it": {
+        "lang": "it-IT",
+        "title": "Accedi",
+        "subtitle": "Inserisci la password per continuare",
+        "placeholder": "Password",
+        "btn": "Accedi",
+        "invalid_pw": "Password non valida",
+        "conn_failed": "Connessione fallita",
+    },
     "ja": {
         "lang": "ja-JP",
         "title": "\u30b5\u30a4\u30f3\u30a4\u30f3",