feat(workspace): opt-in 'start new chat when switching workspace' (#5473)

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>

api/config.py

@@ -8409,6 +8409,7 @@ _SETTINGS_DEFAULTS = {
     "show_conversation_outline": False,  # show opt-in desktop jump-to-question outline panel
     "show_busy_placeholder_hint": False,  # opt-in busy composer placeholder hint
     "hide_empty_state_suggestions": False,  # hide the default new-chat suggestion buttons
+    "new_chat_on_workspace_switch": False,  # #5473 opt-in: switching to a DIFFERENT workspace starts a new chat (leaving the current conversation on its original workspace) instead of mutating the current session's workspace in place. Default OFF preserves the shipped in-place-switch behavior.
     "virtualize_transcript": False,  # #4343: virtualize long (>80 msg) transcripts. EXPERIMENTAL, opt-IN (default OFF). Was opt-out/default-on in #4325 but caused scroll-up flicker on long sessions with tall tool-call rows (variable-height anchor oscillation) — flipped off for everyone in #4343; re-enabling requires an explicit opt-in (see virtualize_transcript_optin migration in load_settings).
     "virtualize_transcript_optin": False,  # #4343 migration marker: True only once the user explicitly enables virtualize_transcript AFTER the default-off flip. A stored virtualize_transcript=True WITHOUT this marker is a stale pre-flip value and is reset to False on load (force-off-for-everyone migration).
     "show_tps": False,  # show tokens-per-second chip in assistant message headers
@@ -8729,6 +8730,7 @@ _SETTINGS_BOOL_KEYS = {
     "show_conversation_outline",
     "show_busy_placeholder_hint",
     "hide_empty_state_suggestions",
+    "new_chat_on_workspace_switch",
     "virtualize_transcript",
     "virtualize_transcript_optin",
     "show_tps",