feat(admin): delete a whitelist entry, and stop hiding a broken mail setup

Three operator gaps, all found the same way — a person could not sign up and the
app showed nobody anything.

- DELETE /api/admin/invites/{id} with a delete control on both the pending
  requests and the previously-decided list. Deny keeps the row so the person
  cannot immediately re-ask; delete forgets the address. Refused with 409 while
  a User row exists for it, because is_allowed reads that row and removing it
  would revoke a live user's access with no other sign — the list ships
  has_account so the UI withholds the button instead of offering one that fails.
- Email is fail-soft by design, which had come to mean invisible. _send() now
  records an email_undelivered notification for every admin when a message is
  skipped or rejected, naming the message that was lost, throttled to one an
  hour. /api/me carries email_configured, driving a standing warning on
  Configuration → Email / SMTP and above the access-request list.
- The sign-in card's Create account / Forgot password / Didn't get the email
  views are history entries (useHistorySubview), so Back returns to the previous
  view instead of only moving the address bar.

Also: the per-test rate-limiter reset sweeps every loaded app.* module rather
than a hand-kept list of three — this change added a limiter outside it.

frontend/src/components/InlineNotice.tsx

@@ -0,0 +1,26 @@
+import type { ReactNode } from "react";
+import type { LucideIcon } from "lucide-react";
+import { AlertTriangle } from "lucide-react";
+
+// An in-flow warning that sits INSIDE a panel, next to the thing it is about — as opposed to
+// Banner, which is the full-bleed top-of-app bar (border-b, app chrome) and looks wrong nested in a
+// card. Same warn tokens as Banner's warning tone so the two read as one system.
+export default function InlineNotice({
+  children,
+  icon: Icon = AlertTriangle,
+  className = "",
+}: {
+  children: ReactNode;
+  icon?: LucideIcon;
+  className?: string;
+}) {
+  return (
+    <div
+      role="status"
+      className={`flex items-start gap-2 rounded-xl border border-warn/30 bg-warn/15 px-3 py-2 text-sm text-fg ${className}`}
+    >
+      <Icon className="w-4 h-4 shrink-0 mt-0.5 text-warn" aria-hidden />
+      <span className="min-w-0">{children}</span>
+    </div>
+  );
+}