chore: upgrade tooling stack

- TypeScript 3.9 -> 5.8
- ESLint 6 -> 9 (flat config)
- Prettier 1 -> 3
- eslint-plugin-vue 6 -> 9
- Add vue-tsc for type checking
- Migrate Sass @import to @use syntax
- Fix Pinia persistence config for v4
- Add Spotify type declarations
- Remove unused registerServiceWorker.ts

src/views/Home.vue

@@ -101,8 +101,6 @@ onMounted(() => {
 </script>
 
 <style lang="scss" scoped>
-@import '@/styles/variables';
-
 .home {
   display: flex;
   flex: 1;