Add glass-filling difficulty visualization for song sections

[carochacs]

Overview

Currently, song sections do not visually communicate their relative difficulty level or how much of that difficulty is being displayed. This feature should implement a glass-filling style display that shows:

• Difficulty percentage for each section (how challenging that section is)
• Visual feedback via different sized "glasses" to represent difficulty levels
• Consistent UI across both the difficulty maker and the playing interface

Goals

• Display each section with a glass-filling metaphor where the fill level represents difficulty
• Vary glass size based on the inherent difficulty of each section
• Make difficulty progression more immediately visible to users
• Improve UX for both creating and playing content

Affected Areas

• Difficulty maker — show difficulty levels when creating/editing sections
• Player UI — display difficulty indicators during playback
• Visualization — implement glass-filling animation/rendering in canvas or DOM

Acceptance Criteria

• Sections display visual difficulty representation (glass-filling style)
• Glass size or fill percentage reflects section difficulty
• UI updates are consistent across difficulty maker and player
• Visual implementation is responsive and aligns with existing Slopsmith UI patterns
• Code follows Slopsmith conventions (Tailwind, canvas patterns, plugin architecture)

This issue becomes relevant only when this plugin is live