feat: migrate spaced themes to CSS variables and add interactive theme builder

[mathuo]

Summary

• Migrate spaced theme visual properties to CSS custom properties for full runtime customizability
• Rename --dv-group-gap-border → --dv-floating-group-border (scoped correctly to floating groups only)
• Add new theme variables: --dv-floating-group-dragging-opacity, --dv-drag-over-border
• Remove dead --dv-group-border-radius variable (was silently overridden by !important, had no effect)
• Delete unused _sash-handle-mixin.scss
• Add interactive theme builder side panel to the demo with live CSS override controls and code export

Test plan

• Verify spaced themes render correctly (padding, border-radius, gaps)
• Verify drag-over overlay shows border on spaced themes
• Verify floating group dragging opacity applies
• Open theme builder panel in demo, change CSS overrides, verify live updates
• Export code snippet from theme builder, verify it's valid
• Switch base themes in demo, verify theme builder reads correct computed defaults

🤖 Generated with Claude Code

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Sandbox	Source
dockview-app	Configuration
editor-gridview	Configuration
externaldnd-dockview	Configuration
fullwidthtab-dockview	Configuration
iframe-dockview	Configuration
keyboard-dockview	Configuration
nativeapp-dockview	Configuration
rendering-dockview	Configuration

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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