Fix missing .json file extension when exporting notes

The UI dialog says "Filename (without .json)" and the default value
doesn't have a .json suffix, so we are supposed to automatically
append the file extension (as we already do for automatic backups).

Fixes FossifyOrg#13.

app/src/main/kotlin/org/fossify/notes/activities/SettingsActivity.kt

@@ -301,7 +301,7 @@ class SettingsActivity : SimpleActivity() {
     private fun setupNotesExport() {
         binding.settingsExportNotesHolder.setOnClickListener {
             ExportNotesDialog(this) { filename ->
-                saveDocument.launch(filename)
+                saveDocument.launch("$filename.json")
             }
         }
     }