papers/platter: move dashboard to /platter/jeffrey/, add platter index
- new /platter/ namespace lists per-platter dashboards (jeffrey now,
whistlegraph soon); lets the URL space breathe as more platters add
visual surfaces
- /platter/jeffrey/ retitled "jeffrey's instagram" with breadcrumb back
to the platter index; entry on /platter/ uses a recent thumb as cover
- Caddyfile try_files now covers both `/jeffrey` (no slash →
{path}/index.html) and `/jeffrey/` (slash → {path}index.html); without
the no-slash term the bare /jeffrey URL fell through to the SPA root
- build-thumbnails.mjs writes manifest to the new location
- platter README updated with new URL
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>