Replace breadcrumbs with back links; keep the zone header on tab switches

The zone page's breadcrumb (Zones › view) linked the view to a filtered
zone list while the view link below it opened the view. It is now a
single "← Zones" link back to the list with its filters, and a changeset
has "← History". View links carry where they came from, so the views page
offers "← example.org (external)" or "← Zones" (only the UI's own #/
addresses are accepted).

Switching a zone's tabs no longer blanks the page: the header and tabs
show at once and the previous tab stays, faded, until the next has
loaded. The router shows "Loading…" only for pages that take a while.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/style.css

@@ -388,9 +388,11 @@ button.chip:hover { background: var(--accent-soft); border-color: var(--accent);
   header.top .brand:hover .logo { transform: none; }
 }
 
-/* breadcrumbs above detail pages */
-.crumbs { display: flex; flex-wrap: wrap; gap: 6px; align-items: baseline; font-size: 13px; color: var(--muted); margin-bottom: 2px; }
-.crumbs .sep { opacity: 0.6; }
+/* a zone tab's previous content while the next tab loads */
+.zone-body.stale { opacity: 0.5; transition: opacity 0.15s 0.1s; pointer-events: none; }
+
+/* the "← Zones" link above a page's heading */
+.back-link { font-size: 13px; margin-bottom: 2px; }
 
 /* view colours (the default is the link colour) */
 .vc-green { color: var(--vc-green); } .vc-teal { color: var(--vc-teal); } .vc-purple { color: var(--vc-purple); }