Three admin templates:
- index: invoice list with status badge and link to detail.
- new: client + terms fields, dynamic line-item rows backed by
admin-invoice-form.js (add/remove + re-index of items[N][...] names).
- show: invoice detail, public URL to share, toggle-paid button.
One public template, invoice_public.erb, shown at /i/:uuid: client name,
number, total, status, and a single big download button to /i/:uuid/pdf.
All admin pages are noindex; same for the public landing (it's a
client-specific URL, not search-discoverable content).
