style(react-ui): adjust max-width and padding for conversation starter component

[i-subham23]

Fixes the suggestion-prompt (conversation starter) row's padding and alignment across breakpoints.

Problem

Two related geometry issues, measured in the openui-cloud example:

1. Narrow widths (content column under 880px): the starter container was max-width: 880px with no horizontal padding, so the chips sat flush against the content edge — 0px gutter — while the composer input below kept its inset.
2. Wide widths: the starter row was based on the 880px thread column while the composer input uses the narrower 768px column, so once both hit max-width they centered independently and the chip row started ~56px left of the input's edge.

Change

conversationStarter.scss only:

• Add the same $space-m-l horizontal gutter the thread and composer already use, with max-width compensated by 2× the gutter so the content width is unchanged when uncapped.
• Base the container on the composer's 768px column (see components/composer.scss) instead of the thread's 880px — the chips belong to the input, so the row now tracks the input's edges at every desktop width.

Test Plan

• Playwright sweep across 20 viewport widths (360–2560) against the openui-cloud example:
  • chip gutter never below 16px on desktop (was 0px flush at 820–1050)
  • chip row's left edge aligns with the composer input's edge (delta 0) at every desktop width — was −16px when squeezed and −56px when wide
  • mobile layout unchanged (its own 18px padding override still wins)
  • no horizontal overflow, no chip clipping, clean behavior through the mobile↔desktop flip and the max-width transition

[ankit-thesys]

Looks good and tested