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/app.js

@@ -49,7 +49,7 @@ const navItems = [
 ];
 
 const app = document.getElementById("app");
-let mainEl, navEl, barEl, current = { key: "" };
+let mainEl, navEl, barEl, current = { key: "" }, routeGen = 0;
 
 function parseHash() {
   const raw = location.hash.replace(/^#\/?/, "");
@@ -162,11 +162,19 @@ async function route() {
     current = { key: path, nav: r.nav, page: r.page, params, query };
     if (!r.params && r.nav) lastList[r.nav] = location.hash;
     renderNav();
-    mount(mainEl, h("div", { class: "muted" }, "Loading…"));
+    // Keep the old page until the new one is drawn (switching a zone's
+    // tabs shouldn't blank the page); say "Loading…" only when it's slow.
+    // (Only while the new page has drawn nothing yet: pages that draw
+    // their frame first fill it in afterwards.)
+    const gen = ++routeGen;
+    const old = mainEl.firstChild;
+    const slow = setTimeout(() => { if (gen === routeGen && mainEl.firstChild === old) mount(mainEl, h("div", { class: "muted" }, "Loading…")); }, 300);
     try {
       await r.page.render(mainEl, params, query);
     } catch (e) {
-      mount(mainEl, h("h1", null, "Something went wrong"), errorBox(e));
+      if (gen === routeGen) mount(mainEl, h("h1", null, "Something went wrong"), errorBox(e));
+    } finally {
+      clearTimeout(slow);
     }
     return;
   }