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/Welcome.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useRef, useState } from "react";
+import { useHistorySubview, type SubviewUrl } from "../lib/history";
 import { useTranslation } from "react-i18next";
 import {
   ArrowRight,
@@ -27,7 +28,19 @@ import { inputCls } from "./ui/form";
 const EMAIL_RE = /^[^@\s]+@[^@\s]+\.[^@\s]+$/;
 const PASSWORD_MIN = 10;
 
-type Mode = "signin" | "register" | "forgot" | "reset" | "resend";
+export type Mode = "signin" | "register" | "forgot" | "reset" | "resend";
+
+// The card's sub-views are real history entries, so browser/mouse Back leaves one the way it leaves
+// any other page — they used to be plain state, and Back silently changed only the address bar.
+// "signin" is the root (no param) and "reset" is deliberately NOT mirrored: that view is only
+// reachable with the one-time token from the #reset fragment, so a bookmarked ?m=reset would open a
+// form that cannot submit.
+export const MODE_URL: SubviewUrl<Mode> = {
+  key: "m",
+  encode: (m: Mode) => (m === "signin" || m === "reset" ? null : m),
+  decode: (raw: string) =>
+    raw === "register" || raw === "forgot" || raw === "resend" ? (raw as Mode) : null,
+};
 
 // Per-mode i18n keys for the card title and the submit button. A flat map (vs nested ternaries)
 // keeps adding a new mode a one-line change and makes a missing key a compile error.
@@ -328,7 +341,10 @@ function AuthCard() {
       .catch(() => setVerify("invalid"));
   }, [verifyToken]);
 
-  const [mode, setMode] = useState<Mode>(resetToken ? "reset" : "signin");
+  const { view: mode, open: setMode } = useHistorySubview<Mode>(
+    resetToken ? "reset" : "signin",
+    MODE_URL,
+  );
   const [email, setEmail] = useState("");
   const [password, setPassword] = useState("");
   const [busy, setBusy] = useState(false);
@@ -344,6 +360,13 @@ function AuthCard() {
       .catch(() => {});
   }, []);
   const [done, setDone] = useState<string>(""); // success/info message replacing the form
+  // `done` and `error` belong to the view that produced them, and the render is `done ? msg : form`
+  // — without this, Back out of a submitted form would show the old success message under the
+  // previous view's title. Clearing on view change covers both Back/Forward and the mode links.
+  useEffect(() => {
+    setDone("");
+    setError("");
+  }, [mode]);
 
   // Explicit demo entry (replaces the old hidden probe): a labelled email field, still gated
   // by the demo whitelist server-side.