feat: add QR code device pairing for mobile app login

app/api/auth/pair/create/route.ts

@@ -0,0 +1,88 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { cookies } from 'next/headers';
+import { logger } from '@/lib/logger';
+import { refreshTokenCookieName, refreshTokenServerCookieName } from '@/lib/oauth/tokens';
+import { buildOAuthParams, getRequiredConfig, getTokenEndpoint } from '@/lib/oauth/token-exchange';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
+import { createPairing } from '@/lib/auth/pairing-store';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
+
+// Desktop side of the cross-device QR login. The caller must be a signed-in
+// webmail session (its refresh token lives in the httpOnly jmap_rt cookie). We
+// refresh that token to (a) prove the session is live and (b) obtain a fresh
+// access token to hand the phone, then stash the bundle under a one-time
+// pairing code. The desktop renders the returned code as a QR; the phone
+// redeems it at /api/auth/pair/redeem.
+//
+// Token sharing note: the phone receives the SAME refresh token as the desktop.
+// That is correct for OAuth servers (such as Stalwart in its default config)
+// that do not rotate refresh tokens on use. If the server rotates refresh
+// tokens, the two devices would fight over the latest token — such deployments
+// should disable rotation for this client or use a token-exchange grant.
+export async function POST(request: NextRequest) {
+  const cookieStore = await cookies();
+  try {
+    const body = await request.json().catch(() => ({}));
+    const slot =
+      typeof body.slot === 'number' && body.slot >= 0 && body.slot < MAX_ACCOUNT_SLOTS
+        ? body.slot
+        : 0;
+
+    const cookieName = refreshTokenCookieName(slot);
+    const refreshToken = cookieStore.get(cookieName)?.value;
+    if (!refreshToken) {
+      return NextResponse.json({ error: 'Not signed in' }, { status: 401 });
+    }
+    const serverId = cookieStore.get(refreshTokenServerCookieName(slot))?.value || null;
+
+    const tokenEndpoint = await getTokenEndpoint(serverId);
+    const params = buildOAuthParams({ grant_type: 'refresh_token', refresh_token: refreshToken }, serverId);
+
+    const tokenResponse = await fetch(tokenEndpoint, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+      body: params.toString(),
+    });
+
+    if (!tokenResponse.ok) {
+      const errorText = await tokenResponse.text();
+      logger.warn('Pair create: refresh failed', { status: tokenResponse.status, error: errorText });
+      // Stale session — clear the dead cookie so the user is prompted to log
+      // back in, mirroring the token route's behaviour.
+      cookieStore.delete(cookieName);
+      cookieStore.delete(refreshTokenServerCookieName(slot));
+      return NextResponse.json({ error: 'Session expired' }, { status: 401 });
+    }
+
+    const tokens = await tokenResponse.json();
+    if (!tokens.access_token) {
+      logger.error('Pair create: refresh response missing access_token');
+      return NextResponse.json({ error: 'Invalid token response' }, { status: 502 });
+    }
+
+    // If the server rotated the refresh token, persist the new one back to the
+    // desktop's cookie so this very session keeps working. The phone will get
+    // the same (new) token below.
+    const effectiveRefreshToken = tokens.refresh_token || refreshToken;
+    if (tokens.refresh_token) {
+      cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
+    }
+
+    const { clientId, serverUrl } = getRequiredConfig(serverId);
+
+    const { code, expiresIn } = createPairing({
+      accessToken: tokens.access_token,
+      refreshToken: effectiveRefreshToken,
+      expiresIn: tokens.expires_in,
+      tokenEndpoint,
+      clientId,
+      serverUrl,
+      serverId,
+    });
+
+    return NextResponse.json({ pairing_code: code, server_url: serverUrl, expires_in: expiresIn });
+  } catch (error) {
+    logger.error('Pair create error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}