feat: Implement note export functionality

Implemented note export functionality, including a formatting function in the ViewModel and UI integration in MainActivity.

app/src/main/java/de/lxtools/noteshop/MainActivity.kt

@@ -153,6 +153,20 @@ fun AppShell(
         }
     )
 
+    val noteExportLauncher = rememberLauncherForActivityResult(
+        contract = ActivityResultContracts.CreateDocument("text/plain"),
+        onResult = { uri ->
+            uri?.let {
+                notesViewModel.noteDetails.value.toNote().let { note ->
+                    val content = notesViewModel.formatNoteForExport(note)
+                    context.contentResolver.openOutputStream(it)?.use { outputStream ->
+                        outputStream.write(content.toByteArray())
+                    }
+                }
+            }
+        }
+    )
+
     ModalNavigationDrawer(
         drawerState = drawerState,
         drawerContent = {
@@ -296,6 +310,26 @@ fun AppShell(
                                         }
                                     )
                                 }
+                            } else if (currentScreen == Screen.NoteDetail) {
+                                var showMenu by remember { mutableStateOf(false) }
+                                IconButton(onClick = { showMenu = !showMenu }) {
+                                    Icon(Icons.Default.MoreVert, contentDescription = "More")
+                                }
+                                DropdownMenu(
+                                    expanded = showMenu,
+                                    onDismissRequest = { showMenu = false }
+                                ) {
+                                    DropdownMenuItem(
+                                        text = { Text(stringResource(R.string.export)) },
+                                        onClick = {
+                                            showMenu = false
+                                            notesViewModel.noteDetails.value.toNote().let { note ->
+                                                val fileName = "${note.title}.txt"
+                                                noteExportLauncher.launch(fileName)
+                                            }
+                                        }
+                                    )
+                                }
                             }
                         },
                         colors = TopAppBarDefaults.topAppBarColors(

app/src/main/java/de/lxtools/noteshop/ui/notes/NotesViewModel.kt

@@ -100,6 +100,13 @@ class NotesViewModel(private val noteshopRepository: NoteshopRepository) : ViewM
         }
     }
 
+    fun formatNoteForExport(note: Note): String {
+        val builder = StringBuilder()
+        builder.appendLine("Title: ${note.title}")
+        builder.appendLine("Content: ${note.content}")
+        return builder.toString()
+    }
+
     companion object {
         private const val TIMEOUT_MILLIS = 5_000L
     }