feat(chat): Replace button with shared component

[kaapi-agent-bot]

Issue

Closes #261

Summary

• Replaces the hand-rolled outline-style <button> for "New chat" with the shared Button component.
  • Uses the default primary variant and sm size.
  • Renders as a solid accent background instead of a faint outline.
• Ensures consistency with the app's UI.

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran npm run dev and npm run build in the repository root and test.
• If you've fixed a bug or added code that is tested

Original PR description

Summary

• Replaces the hand-rolled outline-style <button> for "New chat" in the Chat page header with the shared Button component (app/components/ui/Button.tsx), using its default primary variant and sm size.
• The button now renders as a proper primary action (solid accent background) instead of a faint outline, consistent with the rest of the app's UI.

Closes #261

Test plan

• npm run lint passes with no new errors
• npm run build passes
• Manually verify in the browser that the "New chat" button appears with the primary style and still triggers handleNewChat correctly

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[Ayush8923]

So this PR is generated by the Kaapi-Agent, will merge if everything good.