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

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

web/src/pages/agent/chat/box.tsx

@@ -127,6 +127,7 @@ function AgentChatBox() {
             disabled={isWaitting}
             sendDisabled={sendLoading || isWaitting}
             isUploading={loading || isWaitting}
+            resize="vertical"
             onPressEnter={handlePressEnter}
             onInputChange={handleInputChange}
             stopOutputMessage={stopOutputMessage}

web/src/pages/agent/share/index.tsx

@@ -193,6 +193,7 @@ const ChatContainer = () => {
                   value={value}
                   disabled={hasError || isWaitting}
                   sendDisabled={sendDisabled || isWaitting}
+                  resize="vertical"
                   conversationId={conversationId}
                   onInputChange={handleInputChange}
                   onPressEnter={handlePressEnter}