fix(auth): close the account pre-emption + wrong-account link holes (E1 S5, SEC-10/11)

SEC-10 (account pre-emption / "pre-hijack"): a password set on an account whose
email was never verified is untrusted — whoever registered the address (to
pre-empt the real owner) could have chosen it. Both the Google-adoption callback
and admin activation (_activate_user_by_email) now drop that pre-set password and
bump the session epoch when the account isn't email_verified, so a later Google
login / admin approval can't hand the pre-set password a working login. No-op for
a verified account (the legit no-SMTP flow). +2 tests.

SEC-11 (link/upgrade bound to the session default): the OAuth link/upgrade GETs
resolved to the session DEFAULT account, so a tab acting as a secondary account
would link Google / the YouTube grant to the wrong account. The frontend now
threads the active account through the navigation URL (withActiveAccount ->
?account=<active>), and _complete_link accepts any wallet account, not only the
default, so the identity lands on the account that started the flow.

frontend/src/components/SettingsPanel.tsx

@@ -1,4 +1,5 @@
 import { useState } from "react";
+import { withActiveAccount } from "../lib/activeAccount";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
@@ -418,7 +419,7 @@ function SignInMethods({ me }: { me: Me }) {
           ) : (
             <button
               onClick={() => {
-                window.location.href = "/auth/link";
+                window.location.href = withActiveAccount("/auth/link");
               }}
               className="shrink-0 glass-card glass-hover px-3 py-1.5 rounded-xl text-sm transition"
             >
@@ -644,7 +645,7 @@ function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
               grantedHint={t("settings.account.readGrantedHint")}
               enableHint={t("settings.account.readEnableHint")}
               onEnable={() => {
-                window.location.href = "/auth/upgrade?access=read";
+                window.location.href = withActiveAccount("/auth/upgrade?access=read");
               }}
             />
             <AccessRow
@@ -653,7 +654,7 @@ function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
               grantedHint={t("settings.account.writeGrantedHint")}
               enableHint={t("settings.account.writeEnableHint")}
               onEnable={() => {
-                window.location.href = "/auth/upgrade?access=write";
+                window.location.href = withActiveAccount("/auth/upgrade?access=write");
               }}
             />
           </div>

frontend/src/lib/activeAccount.ts

@@ -19,6 +19,17 @@ export function activeAccountId(): number | null {
   }
 }
 
+// Append `?account=<active>` to a top-level navigation URL (SEC-11). XHRs carry the active account in
+// the X-Siftlode-Account header, but a full-page navigation (window.location) can't, so the OAuth
+// link/upgrade GETs would resolve to the SESSION DEFAULT account instead of the tab's active one — the
+// identity/token would then land on the wrong account. This threads the active account through the URL
+// so resolved_user_id picks it. No-op when there's no distinct active account pinned yet.
+export function withActiveAccount(path: string): string {
+  const id = activeAccountId();
+  if (id == null) return path;
+  return `${path}${path.includes("?") ? "&" : "?"}account=${id}`;
+}
+
 // Reactive active-account. setActiveAccount/clearActiveAccount write sessionStorage — a silent op that
 // triggers no React render — so this tab's per-account state (sort/filter/view/layout, all keyed by the
 // account) would never re-read once the account is finally known. That bites exactly once, on first

frontend/src/lib/onboarding.ts

@@ -12,6 +12,7 @@
 //                                       suppresses the auto-popup on future logins (they
 //                                       can still reopen it from Settings → Account).
 
+import { withActiveAccount } from "./activeAccount";
 import { getAccountRaw, LS, setAccountRaw } from "./storage";
 
 const ONBOARD_ACTIVE = "siftlode.onboarding.active"; // sessionStorage (not in LS, which is localStorage)
@@ -36,7 +37,7 @@ export function shouldAutoOpenOnboarding(me: {
 /** Mark a grant as in progress, then send the user to Google's consent screen. */
 export function beginGrant(access: "read" | "write"): void {
   sessionStorage.setItem(ONBOARD_ACTIVE, "1");
-  window.location.href = `/auth/upgrade?access=${access}`;
+  window.location.href = withActiveAccount(`/auth/upgrade?access=${access}`);
 }
 
 /** Leave the wizard. `dismiss` suppresses the future auto-popup (used when read is skipped). */