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/pages/history.js

@@ -1,4 +1,4 @@
-import { h, mount, table, toast, confirmDialog, diffView, time, fmtTime, badge, plural, crumbs, lastList } from "../ui.js";
+import { h, mount, table, toast, confirmDialog, diffView, time, fmtTime, badge, plural, backLink, lastList } from "../ui.js";
 import { api, get, useDraft } from "../api.js";
 
 export async function render(el, params) {
@@ -30,7 +30,7 @@ async function detail(el, id) {
   mount(el,
     h("div", { class: "page-head" },
       h("div", { class: "grow" },
-        crumbs(["History", lastList.history || "#/history"]),
+        backLink("History", lastList.history || "#/history"),
         h("h1", null, "Changeset #" + c.id),
         h("div", { class: "muted" }, c.description || "(no description)")),
       h("div", { class: "actions" },