build(deps-dev): upgrade vite to 8.1.3 and vite-plugin-svelte to 7.1.2

The two dependabot PRs (#237, #239) each fail alone: plugin-svelte 6
peers on vite ^7 and plugin-svelte 7 peers on vite ^8, so the majors
have to move together.

Config migration for vite 8 (rolldown/oxc):
- esbuild.drop no longer exists; prod console stripping and
  minification move to rolldown's oxc minifier via
  rollupOptions.output.minify (verified: 0 console.* calls in the
  minified prod bundle)
- vite-plugin-svelte 7 removed the hot option (Svelte 5 HMR is
  automatic); it was already disabled under vitest

Author: chhoumann