feat(web): add scroll to last message button

Add intelligent floating scroll button for long conversations that:
- Only appears when scrolling down (direction-aware)
- Auto-hides after 3 seconds of inactivity
- Stays visible on hover to prevent accidental disappearance
- Uses consistent design patterns with repo styling
- Includes proper accessibility features

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Co-Authored-By: Jeremy Osih <osih.jeremy@gmail.com>
Co-Authored-By: opencode <noreply@opencode.ai>