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

Songs can be reordered via drag-and-drop
Reordered list is used when building (overrides config order)
Visual indicator shows drop target (highlight)
Order can be reset via a button
Reordering only enabled when songs.order is "manual"
When alphabetical, list shows alphabetical order and drag is disabled (with hint)
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.

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