feat: add drag-and-drop song reordering in the GUI (Closes #19) (#22)

app/src/main/kotlin/de/pfadfinder/songbook/app/SongbookPipeline.kt

@@ -21,7 +21,15 @@ data class BuildResult(
 
 class SongbookPipeline(private val projectDir: File) {
 
-    fun build(): BuildResult {
+    /**
+     * Build the songbook PDF.
+     *
+     * @param customSongOrder Optional list of song file names in the desired order.
+     *   When provided, songs are sorted to match this order instead of using the
+     *   config-based sort (alphabetical or manual). Files not in this list are
+     *   appended at the end.
+     */
+    fun build(customSongOrder: List<String>? = null): BuildResult {
         // 1. Parse config
         val configFile = File(projectDir, "songbook.yaml")
         if (!configFile.exists()) {
@@ -55,7 +63,7 @@ class SongbookPipeline(private val projectDir: File) {
 
         logger.info { "Found ${songFiles.size} song files" }
 
-        val songs = mutableListOf<Song>()
+        val songsByFileName = mutableMapOf<String, Song>()
         val allErrors = mutableListOf<ValidationError>()
 
         for (file in songFiles) {
@@ -65,7 +73,7 @@ class SongbookPipeline(private val projectDir: File) {
                 if (songErrors.isNotEmpty()) {
                     allErrors.addAll(songErrors)
                 } else {
-                    songs.add(song)
+                    songsByFileName[file.name] = song
                 }
             } catch (e: Exception) {
                 allErrors.add(ValidationError(file.name, null, "Parse error: ${e.message}"))
@@ -76,10 +84,20 @@ class SongbookPipeline(private val projectDir: File) {
             return BuildResult(false, errors = allErrors)
         }
 
-        // Sort songs
-        val sortedSongs = when (config.songs.order) {
-            "alphabetical" -> songs.sortedBy { it.title.lowercase() }
-            else -> songs  // manual order = file order
+        val songs = songsByFileName.values.toList()
+
+        // Sort songs: custom order takes priority, then config-based sort
+        val sortedSongs = if (customSongOrder != null) {
+            val orderMap = customSongOrder.withIndex().associate { (index, name) -> name to index }
+            songs.sortedBy { song ->
+                val fileName = songsByFileName.entries.find { it.value === song }?.key
+                orderMap[fileName] ?: Int.MAX_VALUE
+            }
+        } else {
+            when (config.songs.order) {
+                "alphabetical" -> songs.sortedBy { it.title.lowercase() }
+                else -> songs  // manual order = file order
+            }
         }
 
         logger.info { "Parsed ${sortedSongs.size} songs" }