Shared scroll state support

[Emmaccen]

Added optional singleton mode to useScrollToBottom hook, allowing scroll state and controls to be shared across multiple components without creating duplicate instances.

Why this change:

• Please see: fix: PromptInputTextarea now actually handles its own damn resize logic #1321
• Scroll-to-bottom button position is static and doesn't account for MultimodalInput's config/resize-ability
• Previously, if you wanted to use the scroll-to-bottom button in a separate component (like MultimodalInput) from where the scrollable container lives (like MessagesContainer), you'd have to re-call the hook, which would create a new instance with its own state. This meant the button wouldn't reflect the actual scroll position.
• To make the scroller button pegged to a dynamic and config-friendly MultimodalInput size/height, something needed to be done

What's new:

• Added ScrollToBottomProvider context wrapper
• useScrollToBottom now accepts optional singleton boolean parameter
• When singleton=true, hook returns shared instance from provider
• When singleton=false (default), hook works exactly as before
• useMessages hook now takes the optional singleton prop
• scroll-button now animates as seen in v0.dev

Usage:

1. Wrap your chat area with <ScrollToBottomProvider>
2. Use useScrollToBottom(true) in any child component to access shared state
3. Existing code using useScrollToBottom() continues to work unchanged

Benefits:

• Share scroll state across components (scroll button, auto-scroll, etc.)
• No breaking changes - fully backwards compatible
• Opt-in behavior - existing instances remain isolated by default
• Type-safe with helpful error messages

Example:

<ScrollToBottomProvider>
  <MessagesContainer /> {/* assigns refs with useScrollToBottom(true) */}
  <MultimodalInput /> {/* accesses same instance with useScrollToBottom(true) */}
</ScrollToBottomProvider>

This makes it easy to place scroll controls anywhere in the component tree while maintaining a single source of truth for scroll state.

PS: This PR is a supplementary to #1321

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[MrAllgoodWilson]

Inapprove

[MrAllgoodWilson]

I approve

[MrAllgoodWilson]

I approve