fix: thread per-account cookie slot through OAuth flows

The multi-account refresh-token cookie slot wiring was half-implemented:
every account's refresh token ended up on slot 0, so "+ Add Account"
silently clobbered the previous account's `jmap_rt` cookie. On page
refresh, only the most-recently-added account had a working refresh
token; the others bounced to login.

Three coordinated changes:

1. `app/[locale]/login/page.tsx` (handleOAuthLogin): write the next-free
   cookie slot to `sessionStorage['oauth_cookie_slot']` before redirecting
   to the IdP. `loginWithOAuth` already reads this key but it was never
   written, so it always defaulted to 0.

2. `stores/auth-store.ts` (loginWithOAuth): distinguish "no value set"
   (`rawSlot === null`) from "value is 0". Previously
   `parseInt(getItem(...) || '0')` collapsed both cases, making the
   `getNextCookieSlot()` fallback unreachable.

3. `stores/auth-store.ts` (loginWithServerSso) +
   `app/api/auth/sso/complete/route.ts`: pass the slot through the body of
   the POST and use it for `refreshTokenCookieName(slot)`. Same pattern as
   the existing `/api/auth/token POST` that already accepts a slot. The
   server defaults to 0 for back-compat with any caller that omits it.

After the fix, signing in with multiple accounts produces distinct
`jmap_rt`, `jmap_rt_1`, `jmap_rt_2`, ... cookies (matching the cookieSlot
field in account-store) and all accounts survive a page refresh.

Repro before the fix:
- Sign in with one account, refresh — works.
- Click "+ Add Account", sign in with a second account, refresh — second
  account vanishes from the dropdown; switching to the first account in
  the dropdown still shows the second account's identity in the From box.

app/[locale]/login/page.tsx

@@ -7,6 +7,7 @@ import { useTranslations } from "next-intl";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { useAuthStore } from "@/stores/auth-store";
+import { useAccountStore } from "@/stores/account-store";
 import { useThemeStore } from "@/stores/theme-store";
 import { useShallow } from "zustand/react/shallow";
 import { useConfig } from "@/hooks/use-config";
@@ -460,6 +461,16 @@ export default function LoginPage() {
       sessionStorage.setItem("oauth_add_account_mode", "true");
     }
 
+    // Persist the next-free cookie slot so loginWithOAuth (in stores/auth-store.ts)
+    // writes the refresh token to the correct per-account jmap_rt_<slot> cookie.
+    // loginWithOAuth reads this key but it was previously never written, so every
+    // OAuth account collapsed onto slot 0 and clobbered earlier accounts' refresh
+    // tokens. getNextCookieSlot() returns 0 when no accounts exist (correct for
+    // first sign-in) and the lowest unused slot otherwise (correct for "+ Add
+    // Account").
+    const nextSlot = useAccountStore.getState().getNextCookieSlot();
+    sessionStorage.setItem("oauth_cookie_slot", nextSlot.toString());
+
     const authUrl = new URL(oauthMetadata.authorization_endpoint);
     authUrl.searchParams.set("response_type", "code");
     authUrl.searchParams.set("client_id", oauthClientId);