Feat(UI): Replace prompt window resize handle with bottom edge drag handle.

[DustyShoe]

Summary

This PR replaces the native textarea corner resizer with a full-width draggable bottom edge for prompt fields.
This change should improve UX where tiny resizer was inaccessible when scrollbar was active and in general was hard to hit on displays with high resolution i.e. 1440p.

2026-03-18.20-57-26.mp4

Related Issues / Discussions

Closes #8974

QA Instructions

Verify:

1. Hovering over the draggable edge changes the cursor to a vertical resize cursor, and the edge brightness changes as visual feedback.
2. Resizing the upper (positive prompt) window moves the window below.
3. Resizing the lower window affects only that window.
4. Disabling the negative prompt or switching to a model that does not support it does not affect the behavior.
5. Reloading the page or switching tabs does not reset the prompt window height.

Merge Plan

Simple merge

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)