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/api/auth/sso/complete/route.ts

@@ -13,12 +13,18 @@ export async function POST(request: NextRequest) {
   const cookieStore = await cookies();
 
   try {
-    const { code, state } = await request.json();
+    const { code, state, slot: bodySlot } = await request.json();
 
     if (!code || !state) {
       return NextResponse.json({ error: 'Missing code or state' }, { status: 400 });
     }
 
+    // Per-account refresh-token cookie slot. Without this the route hardcoded
+    // slot 0, so the "+ Add Account" flow overwrote the first account's
+    // refresh-token cookie. Default to 0 for back-compat with any caller that
+    // omits slot. Mirrors the validation in /api/auth/token POST.
+    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : 0;
+
     // Read and decrypt the pending SSO cookie
     const pendingCookie = cookieStore.get(SSO_PENDING_COOKIE)?.value;
     if (!pendingCookie) {
@@ -58,9 +64,9 @@ export async function POST(request: NextRequest) {
     // Exchange code for tokens
     const tokens = await exchangeCodeForTokens(code, codeVerifier, redirectUri);
 
-    // Store refresh token
+    // Store refresh token in the per-account cookie slot.
     if (tokens.refresh_token) {
-      const cookieName = refreshTokenCookieName(0);
+      const cookieName = refreshTokenCookieName(slot);
       cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
     }