# 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`.