fix(gateway): announce declined 'all' scope in /sessions and /resume listings

A non-admin '/sessions all' or '/resume --all' silently downgraded to
chat-scoped listing with zero feedback, which reads as 'my session
vanished' (community Telegram report). Both surfaces now append a notice
that cross-chat listing requires a configured admin. Follows up the
salvaged current-session '(current)' marker (PR #68556, fixes #68547):
sibling tests updated to pin the new contract, new i18n key
gateway.resume.all_requires_admin added to all 17 locales, docs updated.

locales/ga.yaml

@@ -273,6 +273,7 @@ Use `/title My Session` to name the current room session, `/resume --all` to lis
 Future messages in this room will use that transcript until `/reset` or another `/resume`.{msg_part}"
     blocked_not_owner:     "⚠️ /resume blocked: '**{name}**' belongs to a different user or chat. You can only resume sessions from this chat."
     no_named_sessions:     "Níor aimsíodh aon seisiún ainmnithe.\nÚsáid `/title M'Ainm Seisiúin` chun do sheisiún reatha a ainmniú, ansin `/resume M'Ainm Seisiúin` chun filleadh air níos déanaí."
+    all_requires_admin: "_Nóta: teastaíonn riarthóir cumraithe le `--all` (liostú traschomhrá); ní thaispeántar ach seisiúin an chomhrá seo._"
     list_header:           "📋 **Seisiúin Ainmnithe**\n"
     list_item:             "• **{title}**{preview_part}"
     list_item_numbered:    "{index}. **{title}**{preview_part}"