fix(ui): make useAutoScroll smooth flag effective

[Aias00]

Related issue

Closes #2014

Summary

useAutoScroll.scrollToBottom computed the scroll behavior as
smooth ? 'auto' : 'auto'. Both branches returned 'auto', so the
smooth flag had no effect and smooth scrolling was never applied.
Changed the truthy branch to 'smooth' so smooth ? 'smooth' : 'auto'
behaves as intended.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results: npx tsc --noEmit -p chat2db-community-client/tsconfig.json — no errors reported for hooks/useAutoScroll.ts.
• Manual verification: A string literal swap; 'smooth' is a valid ScrollBehavior value per the DOM ScrollToOptions.behavior type.
• UI evidence: N/A

Risk and compatibility

• Public API or stored data: N/A.
• Database or driver compatibility: N/A.
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: N/A.
• Backward compatibility: Smooth scrolling is now actually used when requested; instant scroll still used otherwise.

Reviewer map

• Start here: hooks/useAutoScroll.ts:24 — smooth ? 'auto' : 'auto' -> smooth ? 'smooth' : 'auto'.
• Failure condition: Smooth scrolling still never applied when smooth=true.
• Rollback or disable path: Revert this single commit.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: The fix, verification, and PR description were produced with Claude Code assistance.

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[openai0229]

Reviewed the hook call semantics and scrolling behavior. The change makes the existing smooth flag effective without altering the non-streaming path, and the updated Community frontend checks pass. No blocking issues found.