Allow unsetting widget labels (#6773)

https://github.com/user-attachments/assets/af344318-dac2-4611-b080-910cdfa1e87d

Quick followup to #6752
- Adds support for placeholder values in dialogService.prompt
- When label is unset, initial prompt is empty
- Display original widget name as placeholder
- When prompt returns an empty string (as opposed to null for a canceled
operation), remove widget label

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6773-Allow-unsetting-widget-labels-2b16d73d365081ae9f5dd085d0081733)
by [Unito](https://www.unito.io)

src/components/dialog/content/PromptDialogContent.vue

@@ -4,6 +4,7 @@
       <InputText
         ref="inputRef"
         v-model="inputValue"
+        :placeholder
         autofocus
         @keyup.enter="onConfirm"
         @focus="selectAllText"
@@ -28,6 +29,7 @@ const props = defineProps<{
   message: string
   defaultValue: string
   onConfirm: (value: string) => void
+  placeholder?: string
 }>()
 
 const inputValue = ref<string>(props.defaultValue)