Certificate export, missing PTR list, simpler "Used for"

- Certificates can be exported for use outside Jarl: Export… on the
  Certificates page (download or copy), jarl cert export, and
  GET /certificates/{name}/pem?part=fullchain|cert|key|bundle.
  Administrators only; key exports are audited.
- A reverse zone's missing-PTR banner lists the addresses and names
  (ptr_missing_list in the zone API).
- "Used for" shows DoT/DoH for named, and "not used by Jarl" instead of
  "nothing yet".

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

internal/web/static/assets/pages/zone.js

@@ -67,7 +67,12 @@ export async function render(el, params, query) {
     h("strong", null, "This zone is disabled: "), "named doesn't load it. It is kept here, with its records, and can still be edited; enabling it serves it as it is then.")
     : zone.ptr_missing && can("zones", true) ? h("div", { class: "msg info" },
       plural(zone.ptr_missing, "address", "addresses") + " in " + zone.prefix + " that " + (zone.view ? "view " + zone.view + "'s" : "your") + " A and AAAA records use " + (zone.ptr_missing === 1 ? "has" : "have") + " no PTR record here. ",
-      h("button", { class: "small", onclick: () => fillPTR(zone) }, "Add " + (zone.ptr_missing === 1 ? "it" : "them"))) : null;
+      h("button", { class: "small", onclick: () => fillPTR(zone) }, "Add " + (zone.ptr_missing === 1 ? "it" : "them")),
+      h("ul", { class: "small ptr-missing" }, (zone.ptr_missing_list || []).map((f) => h("li", null,
+        h("span", { class: "mono" }, f.address), " → ", h("span", { class: "mono" }, display(f.target)),
+        f.from ? h("span", { class: "muted" }, " (from view " + f.from + ")") : null,
+        f.others && f.others.length ? h("span", { class: "muted" }, " (also " + f.others.map(display).join(", ") + ")") : null)),
+        zone.ptr_missing > (zone.ptr_missing_list || []).length ? h("li", { class: "muted" }, "and " + (zone.ptr_missing - zone.ptr_missing_list.length) + " more") : null)) : null;
   const tabBar = h("div", { class: "tabs" }, tabs.map(([k, label]) => h("a", { href: "#/zones/" + id + "/" + k, class: k === tab ? "active" : "" }, label)));
   // The header and tabs show at once. Below them, switching tabs keeps the
   // previous tab's content (faded) until the new one is loaded, rather