Migrate Sass @import to @forward/@use

Created by: jedmund

Summary

• Convert @import to @forward in imports.scss and variables.scss
• Add @use 'variables' to themes.scss so it can reference variables under the module system
• Remove redundant @import '$styles/...' lines from 47 Svelte components (already auto-injected via Vite additionalData)
• Eliminates ~900 Sass @import deprecation warnings

Test plan

• Verify pnpm run build compiles all SCSS without @import deprecation warnings
• Verify pnpm run dev still renders all pages correctly
• Spot-check admin composer, photo views, and geo card styling