fix: disable slug field when editing an existing article

The d-tag is the article's identity in Nostr addressable events.
Changing it on edit would create a new article instead of updating
the existing one. The slug field is now disabled when isEditing is true.

https://claude.ai/code/session_011JuQbdA12WdPxC7aGvUBqJ

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/discover/nip23LongForm/LongFormPostScreen.kt

@@ -324,6 +324,7 @@ private fun MarkdownPostScreenBody(
                             .fillMaxWidth()
                             .padding(horizontal = Size10dp, vertical = Size5dp),
                     singleLine = true,
+                    enabled = !postViewModel.isEditing,
                 )
 
                 // Tags section

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/discover/nip23LongForm/LongFormPostViewModel.kt

@@ -207,6 +207,8 @@ class LongFormPostViewModel :
     // Editing existing article
     var existingDTag: String? = null
 
+    val isEditing: Boolean get() = existingDTag != null
+
     fun init(accountVM: AccountViewModel) {
         this.accountViewModel = accountVM
         this.account = accountVM.account