Add middle stage size (360x270) with 600px min-height support

[takaokouji]

Background

Current minimum screen height is 640px, but target PC minimum is 600px. Need to add a middle stage size to better accommodate 600px height while maintaining usable UI.

Requirements

Phase 1: Reduce Min-Height

• Change min-height: 640px to min-height: 600px in playground/index.css

Phase 2: Add Middle Stage Size

• Add middle size constants (360x270, scale 0.75)
• Update stage size resolution logic
• Add 3rd toggle button (small | middle | large)
• Adjust sprite info x, y labels to appear above inputs for middle size only
• Keep largeConstrained for backward compatibility

Phase 3: Make Middle Default

• Set default stage size to middle
• Verify layout balance at 600px height

Completion Criteria

• Phase 1: Min-height is 600px
• Phase 2: Middle size (360x270) selectable via 3-button toggle
• Phase 2: Sprite info x, y labels above inputs for middle size only
• Phase 3: Middle is default, layout is balanced
• All tests pass
• No layout overflow at 600px height
• Smooth transitions between sizes

Files to Modify

• packages/scratch-gui/src/playground/index.css
• packages/scratch-gui/src/lib/layout-constants.js
• packages/scratch-gui/src/lib/screen-utils.js
• packages/scratch-gui/src/components/stage-header/stage-header.jsx
• packages/scratch-gui/src/components/stage-header/icon--middle-stage.svg (new)
• packages/scratch-gui/src/containers/stage-header.jsx
• packages/scratch-gui/src/reducers/stage-size.js
• packages/scratch-gui/src/components/sprite-info/sprite-info.jsx

Notes

• Minimize conflicts when merging upstream changes
• Use "Smalruby:" comments for all modifications
• Keep largeConstrained (don't delete)
• Follow existing patterns and code style