fix(flags): remove @sveltejs/kit from peer dependencies (#442)

@sveltejs/kit was declared as an optional peer with a "*" range, causing
npm to auto-install the newest @sveltejs/kit and pull in its transitive
@sveltejs/vite-plugin-svelte → vite peer chain. In non-SvelteKit projects
already on Vite 7 (via Vitest, Storybook, etc.) this produced a hard
ERESOLVE error requiring `npm install --force`.

Move @sveltejs/kit to devDependencies so the sveltekit entrypoint still
builds and type-generates in the monorepo. SvelteKit consumers always have
@sveltejs/kit installed as the framework, so flags/sveltekit continues to
resolve it from their own tree.

fixes #440