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>

CHANGELOG.md

@@ -19,10 +19,11 @@ All notable changes to Jarl are documented here. The format follows
   list show its name in that colour. The colours are muted, and pastel in
   dark mode. The default is the link colour. A colour isn't configuration,
   so setting one needs no changeset or deploy.
-- Breadcrumbs on the zone page (Zones › view) and the changeset page
-  (History). The zone list keeps its filter, view and type in the address,
-  so going back to it, by breadcrumb or the browser's back button, restores
-  them.
+- A "← Zones" link above a zone, back to the zone list, and "← History"
+  above a changeset. The zone list keeps its filter, view and type in the
+  address, so going back to it (with that link or the browser's back
+  button) restores them. A view opened from a zone or the zone list has a
+  "← example.com (external)" or "← Zones" link back.
 
 ### Changed
 
@@ -33,10 +34,12 @@ All notable changes to Jarl are documented here. The format follows
   release still showed the release's version. `jarld version`,
   `jarl --version`, `GET /status` and `/auth/whoami` report the same, the
   latter two as `build` (docs/development.md).
-
 - The view in the zone list, and on the zone page, links to that view,
   and the views page highlights it. It used to be plain text, and the zone
   page linked to the views page as a whole.
+- Switching a zone's tabs (Records, Settings, …) no longer blanks the page
+  while the tab loads: the page changes once the tab is ready, and
+  "Loading…" appears only if a page takes a while.
 
 ### Fixed