Chat: Update primary button style

Problem

The "New chat" button in the Chat page header does not look like a proper primary action — it currently renders as a plain <button> with hand-rolled outline-style classes instead of using the shared Button component.

File: app/(main)/chat/page.tsx (~line 464-470)

<button
  type="button"
  onClick={handleNewChat}
  className="px-3 py-1.5 rounded-full text-xs font-medium border border-border bg-bg-primary text-text-primary hover:bg-neutral-50 transition-colors cursor-pointer"
>
  New chat
</button>

This mimics the outline variant of app/components/ui/Button.tsx, making it look faint/secondary.

Expected

Use the shared Button component with the primary (default) variant so "New chat" reads as a proper button, consistent with the rest of the app:

<Button onClick={handleNewChat} size="sm">New chat</Button>

Screenshot (current, faint outline look)

Button top-right blends into the header instead of standing out as a primary action.