Fix: enable chat input resizing (#12998)

## Summary
- add resizable support to shared textarea component
- enable vertical resizing for chat inputs in chat and share surfaces
- preserve autosize behavior while honoring manual resize height

## Test plan
- not run (not requested)

Fixes #12803

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/components/message-input/next.tsx

@@ -56,6 +56,7 @@ interface NextMessageInputProps {
   removeFile?(file: File): void;
   showReasoning?: boolean;
   showInternet?: boolean;
+  resize?: 'none' | 'vertical' | 'horizontal' | 'both';
 }
 
 export function NextMessageInput({
@@ -65,6 +66,7 @@ export function NextMessageInput({
   sendLoading,
   disabled,
   showUploadIcon = true,
+  resize = 'none',
   onUpload,
   onInputChange,
   stopOutputMessage,
@@ -211,6 +213,7 @@ export function NextMessageInput({
           disabled={isUploading || disabled || sendLoading}
           onKeyDown={handleKeyDown}
           autoSize={{ minRows: 1, maxRows: 8 }}
+          resize={resize}
         />
         <div className={cn('flex items-center justify-between gap-1.5')}>
           <div className="flex items-center gap-3">