🎨 style(EditChannel): replace fixed-height TextArea with autosize to eliminate unwanted scrollbar

The “Batch Create” secret-key input in Channel Edit previously used a TextArea
with a hard-coded `minHeight`, which caused an extra scrollbar and blank space
on the right side of the field.
This change:

• Removes the fixed `minHeight` in favour of `autosize={{ minRows: 6, maxRows: 6 }}`
• Keeps the field’s rounded appearance while letting it grow/shrink with
  content, improving usability on both desktop and mobile

No other components or global styles are affected.

web/src/pages/Channel/EditChannel.js

@@ -530,7 +530,7 @@ const EditChannel = (props) => {
                         handleInputChange('key', value);
                       }}
                       value={inputs.key}
-                      style={{ minHeight: 150, fontFamily: 'JetBrains Mono, Consolas' }}
+                      autosize={{ minRows: 6, maxRows: 6 }}
                       autoComplete='new-password'
                       className="!rounded-lg"
                     />