🤖 Auto-grow chat input to 50vh with scroll for long content (#123)

Simplified chat input UX by removing manual resize dragger in favor of
auto-growing textarea based on content.

## Changes

**VimTextArea:**
- Changed max-height from 200px to 50vh
- Updated auto-resize logic to use `window.innerHeight * 0.5`

**ChatInput:**
- Removed ResizeHandle component and all drag logic
- Updated focusMessageInput and editingMessage effects to use 50vh max

## Result

Input grows naturally as you type, scrolls when content exceeds 50% of
viewport height. No manual resizing needed - much simpler UX.

_Generated with `cmux`_

---------

Co-authored-by: Ammar Bandukwala <ammar@ammar.io>

Author: ammar-agent

src/components/ChatInput.tsx

@@ -322,7 +322,7 @@ export const ChatInput: React.FC<ChatInputProps> = ({
       element.selectionStart = cursor;
       element.selectionEnd = cursor;
       element.style.height = "auto";
-      element.style.height = Math.min(element.scrollHeight, 200) + "px";
+      element.style.height = Math.min(element.scrollHeight, window.innerHeight * 0.5) + "px";
     });
   }, []);
 
@@ -358,7 +358,8 @@ export const ChatInput: React.FC<ChatInputProps> = ({
       setTimeout(() => {
         if (inputRef.current) {
           inputRef.current.style.height = "auto";
-          inputRef.current.style.height = Math.min(inputRef.current.scrollHeight, 200) + "px";
+          inputRef.current.style.height =
+            Math.min(inputRef.current.scrollHeight, window.innerHeight * 0.5) + "px";
           inputRef.current.focus();
         }
       }, 0);

src/components/VimTextArea.tsx

@@ -45,7 +45,7 @@ const StyledTextArea = styled.textarea<{
   font-size: 13px;
   resize: none;
   min-height: 32px;
-  max-height: 200px;
+  max-height: 50vh;
   overflow-y: auto;
   caret-color: ${(props) => (props.vimMode === "normal" ? "transparent" : "#ffffff")};
 
@@ -123,7 +123,7 @@ export const VimTextArea = React.forwardRef<HTMLTextAreaElement, VimTextAreaProp
       const el = textareaRef.current;
       if (!el) return;
       el.style.height = "auto";
-      const max = 200;
+      const max = window.innerHeight * 0.5; // 50vh
       el.style.height = Math.min(el.scrollHeight, max) + "px";
     }, [value]);