Add dynamic() support to prompt.form() plugin API

- prompt.form() inputs can now have dynamic() callbacks that update
  reactively when form values change (same pattern as auth/template plugins)
- Changed PromptFormRequest routing from one-shot to bidirectional events
- Added PromptFormResponse.done field to distinguish intermediate updates
- Added optional size (enum) to PromptFormRequest for dialog sizing
- Added optional rows to FormInputEditor for fixed height editors
- New httpsnippet plugin: generates code snippets with dynamic language
  and library selectors that update the code preview in real-time

src-web/components/DynamicForm.tsx

@@ -360,8 +360,9 @@ function EditorArg({
         className={classNames(
           'border border-border rounded-md overflow-hidden px-2 py-1',
           'focus-within:border-border-focus',
-          'max-h-[10rem]', // So it doesn't take up too much space
+          !arg.rows && 'max-h-[10rem]', // So it doesn't take up too much space
         )}
+        style={arg.rows ? { height: `${arg.rows * 1.4 + 0.75}rem` } : undefined}
       >
         <Editor
           id={id}