feat(webmail): account switcher for the several signed-in accounts

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/webmail/messages/en.json

@@ -533,5 +533,13 @@
     "zoom": "Zoom",
     "dragToPosition": "Drag to position",
     "notAnImage": "That file could not be read as an image."
+  },
+  "accounts": {
+    "switchAccount": "Switch account",
+    "addAccount": "Add another account",
+    "signOutOne": "Sign out of {email}",
+    "signOutAll": "Sign out of all accounts",
+    "active": "Active",
+    "settings": "Account settings"
   }
 }