feat: add SvelteKit migration foundation

[glitchedmob]

Adds the SvelteKit 3 foundation so later migration layers can port routes and UI against a working app.

• Replaces the active Next.js toolchain with SvelteKit 3, Svelte 5, Vite 8, adapter-node, TypeScript checks, and Vitest.
• Adds a temporary root route, shared Tailwind globals, Source Sans 3 loading, and private runtime env declarations.
• Preserves the old Next and React source as ignored reference code while moving static assets to SvelteKit's static/ directory.
• Updates CI and Docker to build and smoke the adapter-node output.