Root cause: TerminalInstance.onMounted() called sessions.removeSession()
on the pending session, dropping sessions.length to 0. SessionContainer's
v-if="hasSessions" went false, unmounting the entire terminal UI before
the WebSocket could establish and add the real session.
Fix: Added replaceSession() to session store. TerminalInstance no longer
removes the pending session — instead passes its ID to connectToHost(),
which swaps it in-place when the backend responds with the real session ID.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
