Replaces the four independent per-table project-code dropdowns with a
single global scope carried in the URL as ?site=, resolved server-side,
and applied as a PocketBase site= filter. Defines the getSiteScope()
seam so the separate PocketBase users work plugs in without touching
the 23 Topbar call sites.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>