Salvage-rebuild of #5473 (@ruizanthony), reworked to OPT-IN per maintainer
decision. New default-OFF setting new_chat_on_workspace_switch: when enabled,
selecting a DIFFERENT workspace starts a fresh chat bound to that workspace
(leaving the current conversation on its original workspace, avoiding stale
cross-workspace context) instead of mutating the current session in place.
Same-workspace selection stays an in-place refresh. Default off preserves the
shipped behavior for everyone.
Wiring: config default + bool key; window._newChatOnWorkspaceSwitch set on
boot + autosave; Settings checkbox (Preferences); gated branch in
switchToWorkspace() that runs before the in-place /api/session/update; 3 i18n
keys across all 15 locales; regression tests (default-off, gated-branch-present,
flag-wiring, checkbox+i18n).
Co-authored-by: ruizanthony <ruizanthony@users.noreply.github.com>