The "Copy chat" button was hidden during a brand-new chat's very first
turn: both the `canExport` gate and the `handleCopy` early-return required
an `activeChatId` AND persisted `messageRows`, neither of which exists yet
while the first turn is streaming or after it was interrupted before any
row was persisted.
Decouple the export gate from persisted state. ChatThread now reports a
reactive `onLiveContentChange(messages.length > 0)` signal (the live
snapshot lives in a non-reactive ref, so a separate reactive flag is
needed to re-render the button); the parent keeps it in `hasLiveContent`
and exports whenever there is anything on screen OR persisted. `handleCopy`
passes a `"unsaved"` placeholder chat id when none exists yet, and the
live-first builder serializes the on-screen thread WYSIWYG.
Builds on #160 (WYSIWYG export); covers the first-turn edge case that was
explicitly out of scope there.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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