Update SCSS to fix Dart SCSS deprecation warnings

[Copilot]

This PR updates all SCSS files to use modern Dart SCSS syntax, eliminating deprecation warnings that were being issued by newer SCSS compilers.

Changes Made

1. Fixed Global Built-in Function Usage

• Replaced deprecated global mix() function calls with color.mix()
• Added @use 'sass:color'; import to access the color module

2. Modernized Import System

• Converted all @import statements to @use statements with proper namespacing
• Variables are maintained in _lichess-pgn-viewer.vars.scss (companion to the main lib file)
• Main lib file imports variables and all partials using modern @use syntax
• Partials that need variables import the vars file directly to avoid circular dependencies
• No @forward statements needed

3. Fixed Minor Bug

• Corrected missing $ prefix in _fbt.scss variable reference (lpv-fbt-hover → $lpv-fbt-hover)

Before

Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.
Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use color.mix instead.
Warning: 17 repetitive deprecation warnings omitted.

After

[2025-08-25 16:44] Compiled scss/lichess-pgn-viewer.scss to demo/lichess-pgn-viewer.css.

The generated CSS output remains functionally identical - all color mixing computations produce the same results, and all variables resolve correctly.

Fixes #45.

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