fix(UI): keep resize handle out of prompt textarea

[DustyShoe]

Summary

Fix prompt textarea layout so the resize handle no longer covers the last visible line of text when the prompt scrolls during caret navigation with the Up and Down arrow keys. This keeps the existing resize behavior, but reserves space below the prompt content in invokeai/frontend/web/src/features/parameters/components/Core/ParamPositivePrompt.tsx and invokeai/frontend/web/src/features/parameters/components/Core/ParamNegativePrompt.tsx, using a shared height constant from invokeai/frontend/web/src/features/parameters/components/Prompts/PromptResizeHandle.tsx.

Related Issues / Discussions

https://discord.com/channels/1020123559063990373/1149506274971631688/1500867977631764553

QA Instructions

1. Enter enough text to make each textarea scrollable.
2. Place the caret near the end of the prompt and use the Up and Down arrow keys to scroll through the text.
3. Confirm the last visible line stays fully visible above the resize handle.
4. Scroll with the mouse wheel and confirm behavior is unchanged.
5. Drag the resize handle and confirm prompt resizing still works.

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)