Fix LilyPond melody/guide tones staff overlap and web UI null checks

[johnksterling]

Summary

This PR addresses two issues:

1. LilyPond Dual-Staff Formatting Overhaul:

   • Encloses melody and guide tones staves in a \new GrandStaff with explicit StaffGrouper.staff-staff-spacing (padding 3.5, basic distance 11) and system spacing overrides.
   • Eliminates hardcoded page-count = #1 compression when melody is enabled with multiple systems, allowing lead sheets to naturally paginate across pages without vertically colliding staves.
   • Groups simultaneous polyphonic/chord melody notes into chords (<pitch1 pitch2 ...>dur) rather than emitting them sequentially, preventing measure overflow.
   • Adds decomposeDuration and durationToSixteenths with exact sixteenth-unit tracking to guarantee measures always match the time signature duration and properly tie complex values.
   • Supports arbitrary meter full-measure rests (e.g. 5/4, 3/4) and exact skip durations (lilypondSkip) for device spanners.
   • Sorts and deduplicates harmony events across multi-voice MusicXML tracks (e.g. Blue Monk) to preserve correct chord chronological ordering.

2. Web UI & API Null-Safety:

   • Initializes empty non-nil slices for chords, devices, and journeySpans in JSON responses to prevent front-end null.map() runtime exceptions on tunes with unharmonized measures.
   • Adds null-safety guards in ChordsTable, ScoreViewer, and App.
   • Rebuilds frontend bundle in web/dist.

Testing

• go test -v ./... passes all unit and integration tests.
• Tested LilyPond PDF & PNG generation across Autumn Leaves, Take Five (5/4 meter), Blue Monk (multi-voice blues), and Jordu, verifying zero barcheck warnings and clean layout.