fix: address the feature review — accurate copy, reachable controls, real coverage

Round 1 on the feature diff found 15; the substantive ones:

- The delete control VANISHED on any invite whose address had registered, which
  is the ordinary sign-up shape (register writes the User and the pending invite
  together). The guard stays broad — that row is the account's approval gate, and
  orphaning it strands someone who can then neither sign in nor be approved — but
  the control is now a disabled button carrying the reason, on both lists, so an
  admin never sees a row silently missing a button its neighbours have.
- Deleting an entry is not always a revoke: is_allowed falls back to the env
  ALLOWED_EMAILS / ADMIN_EMAILS lists once no row exists, and an ADMIN_EMAILS
  address is force-promoted on its next Google sign-in. The route now reports
  still_allowed_by_env and the UI says so instead of claiming access was removed.
- "Deny keeps the row so they can't immediately re-ask" was simply false —
  upsert_pending_invite reopens a denied row. Corrected everywhere it was stated.
- The Configuration warning reads me.email_configured, which save() did not
  invalidate, so it kept warning about fields the admin had just filled in.
- The read-only variant was an aria-hidden span explained only by a Tooltip that
  the hints toggle can switch off; now a disabled button with title + aria-label.
- The delete error path threw away the 409's reason (the stale-list case) and
  left the stale row on screen.
- The throttle window was spent even when no notice was written, silencing the
  next hour on the path whose job is to stop silent drops.
- email_enabled() opened a second session and decrypted the SMTP password on
  every /api/me poll; it now takes the caller's session.
- welcomeMode.dom.test.ts never rendered the card, so reverting the history
  wiring to useState left it green. A render test now drives the real links and
  fails on that revert (verified).

Plus /api/me flag tests, the registration-shaped pending case, the env-allowlist
case, and a release note that matches what the code does.

frontend/src/components/welcomeMode.render.dom.test.tsx

@@ -0,0 +1,78 @@
+// @vitest-environment jsdom
+//
+// The sign-in card's sub-views as the user meets them: clicking a link must leave a history entry,
+// so Back — including the mouse's back button — returns to the previous view. They used to be plain
+// React state, and Back moved only the address bar while the form on screen stayed put. The
+// encode/decode table lives in welcomeMode.dom.test.ts; this file renders the real card, so a revert
+// to useState fails here.
+//
+// i18n is mocked to a key-passthrough so queries read stable keys, and the API module is stubbed —
+// the card probes /auth/config on mount and must not reach the network.
+import { describe, it, expect, afterEach, beforeEach, vi } from "vitest";
+import { render, screen, fireEvent, cleanup, act } from "@testing-library/react";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return {
+    ...actual,
+    useTranslation: () => ({ t: (key: string) => key, i18n: { language: "en" } }),
+  };
+});
+
+vi.mock("../lib/api", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../lib/api")>();
+  return {
+    ...actual,
+    api: { ...actual.api, authConfig: () => Promise.resolve({ google_enabled: false }) },
+  };
+});
+
+import Welcome from "./Welcome";
+
+const back = () =>
+  act(() => {
+    // jsdom's history.back() is async and does not rewind location, so land on the previous entry
+    // the way the browser would and fire popstate. The push itself is asserted via history.length.
+    window.history.replaceState(null, "", "/");
+    window.dispatchEvent(new PopStateEvent("popstate", { state: null }));
+  });
+
+beforeEach(() => window.history.replaceState(null, "", "/"));
+afterEach(() => {
+  cleanup();
+  window.history.replaceState(null, "", "/");
+});
+
+describe("sign-in card sub-views and the Back button", () => {
+  it("opens Forgot password as a history entry and comes back to sign in", async () => {
+    render(<Welcome />);
+    expect(await screen.findByText("welcome.auth.signinTitle")).toBeTruthy();
+    const entries = window.history.length;
+
+    fireEvent.click(screen.getByText("welcome.auth.forgotLink"));
+
+    expect(screen.getByText("welcome.auth.forgotTitle")).toBeTruthy();
+    expect(window.history.length).toBe(entries + 1); // a Back now has somewhere to go
+    expect(new URLSearchParams(window.location.search).get("m")).toBe("forgot");
+
+    back();
+
+    expect(screen.getByText("welcome.auth.signinTitle")).toBeTruthy();
+    expect(screen.queryByText("welcome.auth.forgotTitle")).toBeNull();
+  });
+
+  it("does the same for Create an account and the verification-email link", async () => {
+    render(<Welcome />);
+    await screen.findByText("welcome.auth.signinTitle");
+
+    fireEvent.click(screen.getByText("welcome.auth.createLink"));
+    expect(screen.getByText("welcome.auth.createTitle")).toBeTruthy();
+    back();
+    expect(screen.getByText("welcome.auth.signinTitle")).toBeTruthy();
+
+    fireEvent.click(screen.getByText("welcome.auth.resendLink"));
+    expect(screen.getByText("welcome.auth.resendTitle")).toBeTruthy();
+    back();
+    expect(screen.getByText("welcome.auth.signinTitle")).toBeTruthy();
+  });
+});