Address review: give Chat its own Settings tab instead of a section inside
Preferences, and default the floating window OFF (opt-in).
- New Settings → Chat tab (chat-tab.tsx) under My Account; moves the
floating-window toggle out of the Preferences tab.
- floatingChatEnabled now defaults OFF — only an explicit enable from the
Chat tab mounts the FAB/overlay.
- i18n: page.tabs.chat + a top-level chat block (en/zh-Hans/ja/ko);
revert the Preferences chat section and its test mock; store tests updated
for the opt-in default.
Co-authored-by: multica-agent <github@multica.ai>
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