fix: use nodeType instead of isEnvelope for icon display

Change isEnvelope computed property to check nodeType === 'envelope'
instead of looking for a dedicated isEnvelope field.

This aligns with the backend response structure and ensures envelope
icons are displayed correctly immediately after creation.

Signed-off-by: Vitor Mattos <[email protected]>

Author: vitormattos

src/views/FilesList/FileEntry/FileEntryPreview.vue

@@ -62,7 +62,7 @@ export default {
 			return this.source?.attributes?.favorite === 1
 		},
 		isEnvelope() {
-			return this.source?.isEnvelope === true
+			return this.source?.nodeType === 'envelope'
 		},
 		previewUrl() {
 			if (this.backgroundFailed === true) {