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

Add a ReorderableSongList composable that supports long-press drag-and-drop
reordering with visual drop target highlighting. Drag-and-drop is only
enabled when songs.order is "manual"; alphabetical mode shows a hint.
The custom order is passed to SongbookPipeline via a new customSongOrder
parameter and a reset button restores the original order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.plans/issue-19-drag-and-drop.md

@@ -0,0 +1,41 @@
+# Issue #19: Add drag-and-drop song reordering in the GUI
+
+## Summary
+
+Add drag-and-drop reordering of songs in the GUI song list. When `songs.order` is "manual", users can drag songs to rearrange them. The custom order is passed to the pipeline at build time. When order is "alphabetical", drag-and-drop is disabled with a hint.
+
+## AC Verification Checklist
+
+1. Songs can be reordered via drag-and-drop
+2. Reordered list is used when building (overrides config order)
+3. Visual indicator shows drop target (highlight)
+4. Order can be reset via a button
+5. Reordering only enabled when songs.order is "manual"
+6. When alphabetical, list shows alphabetical order and drag is disabled (with hint)
+7. GUI remains responsive during drag operations
+
+## Implementation Steps
+
+### Step 1: Add customSongOrder parameter to SongbookPipeline.build()
+
+Add an optional `customSongOrder: List<String>? = null` parameter. When provided, use this ordered list of file names to sort the parsed songs instead of the config-based sort.
+
+### Step 2: Create ReorderableSongList composable
+
+Build a song list that supports drag-and-drop reordering:
+- Use `detectDragGesturesAfterLongPress` on each item to detect drag start
+- Track the dragged item index and current hover position
+- Show a visual indicator (highlighted background) at the drop target
+- On drop, reorder the list
+
+### Step 3: Integrate into App.kt
+
+- Track `songsOrder` config value ("alphabetical" or "manual")
+- Track `originalSongs` list (from file loading) to support reset
+- When manual: enable drag-and-drop, show reset button
+- When alphabetical: disable drag-and-drop, show hint
+- Pass custom order (file names) to pipeline on build
+
+### Step 4: Add reset button
+
+"Reihenfolge zurücksetzen" button restores `songs` to `originalSongs`.