Fix auto-scroll behavior when AI assistant streams responses in the web UI.
Previously, the viewport would remain at the sent message position and users
had to manually click a badge to see streaming responses.
Fixes#14959
Changes:
- Reset chat scroll state before sending message to ensure viewport readiness
- Force scroll to bottom after message send to position viewport correctly
- Detect streaming start (chatStream: null -> string) and trigger auto-scroll
- Ensure smooth scroll-following during entire streaming response
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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