Use Design System custom properties in editor styles

[maxelkins]

What changed

• Replaced the copied Design System Sass colour, spacing, font-weight, and line-height variables with the custom properties provided by design-system-core.
• Replaced the local font-size mixins with the official typography mixins.
• Removed the obsolete local token files. Astro Pi and code syntax colours remain editor-owned custom properties because the Design System has no equivalents.
• Applied the token mappings documented in the Design System's spacing migration guide and typography migration guide. Colour variables use the corresponding properties from the Design System colour definitions.

Visual changes

Most replacements resolve to the same value. These cases intentionally or necessarily differ:

• Sidebar panel headings now use style-1-5(bold). This makes them explicitly bold and reduces the font size from 1.5rem to 1.25rem, with the matching Design System line height.
• The settings panel gap now uses --space-3 at 1.5rem. The old 1.25rem value has no single Design System spacing token, and this avoids combining two properties in calc().
• Modal body copy now uses the canonical --line-height-1-5 value. This changes its line height from the old unitless 1.4 value to 1.5rem.
• The legacy danger button hover colour now derives from --rpf-alert-error with a 15% black mix because the old $rpf-alerts-error-tint has no Design System property.
• A few colours may shift slightly where the copied hex values had drifted from the canonical Design System definitions. For example, active tertiary controls now use the current --rpf-teal-900 value.