fix: PromptInputTextarea now actually handles its own damn resize logic

[Emmaccen]

This component was accepting minHeight/maxHeight/disableAutoResize props and doing absolutely nothing with them. Meanwhile, the parent Composer/MultimodalInput had adjustHeight() and resetHeight() functions that both just hardcoded "44px" and called it a day 🥂.

What's fixed:

• Implemented actual autoresize logic inside PromptInputTextarea
• Forward refs properly so parents can access textarea without fighting us
• Actually use the min/max height props we've been accepting
• Implement resizeOnNewLinesOnly prop (it existed but did nothing before)
• Remove manual height manipulation from Composer - let the component handle it
• No breaking changes - all existing props still work
• Miscellaneous

Changes in MultimodalInput:

• Removed adjustHeight() and resetHeight() functions
• Removed manual ref.current.style.height manipulation
• Component now properly uses PromptInputTextarea's built-in resize

Changes in PromptInputTextarea:

• Added forwardRef support
• Implemented proper autoresize with useEffect
• Apply minHeight/maxHeight via inline styles
• Track newline count for resizeOnNewLinesOnly optimization
• Single resize per change (no double execution)

The textarea now grows and shrinks nicely.

This PR will be followed by a fix (New PR) addressing the scroll-to-bottom button that currently doesn't respond to the auto-resizing text area input.

PS: Thank you for all the amazing work on this project. I truly appreciate the efforts of everyone involved and I'm committed to supporting Vercel and all its projects. Your dedication makes a real difference!

Edit:
See #1322 for supplementary fixes

[vercel]

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