SvelteKit Explicit Env Vars

A src/env.ts manifest using defineEnvVars from @sveltejs/kit/env, the import path introduced in 2.70.0 (the older @sveltejs/kit export is now deprecated). Replaces prefix-based $env/static and $env/dynamic with one declared table where every variable states its visibility, whether it is inlined at build time, a description that becomes editor hover documentation, and a Standard Schema validator that runs once at startup so a bad value fails the boot rather than the first request that needs it. Ships dependency-free Standard Schema validators for https origins, bounded integers and Postgres URLs, so env values arrive typed as number or string rather than as raw strings. Covers all four visibility and staticness combinations.