Refactor: switch from bespoke to semantic colour naming
- Replaced bespoke colour variables (--color-sage, --color-mint, --color-jade)
with semantic equivalents (--color-primary, --color-secondary, --color-accent)
across global CSS and all Svelte components.
- Updated all class references to use the new semantic naming convention.
- Ensured visual consistency and accessibility are maintained (WCAG AA compliant).
- Improves readability, scalability, and design-system alignment.
This refactor replaces project-specific colour tokens with semantic ones that
better express intent, simplifying future theming and maintenance.