chore: remove old SPA source files

Delete files replaced by SvelteKit route structure:
- src/App.svelte, src/main.js (replaced by +layout.svelte + routes)
- src/components/Nav.svelte (replaced by src/lib/Nav.svelte)
- src/pages/*.svelte (replaced by src/routes/**/+page.svelte)
- index.html (replaced by src/app.html)
- public/404.html (SvelteKit adapter-static handles 404 natively)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ccross2