feat(web): add desktop callback API route

Server-side redirect to deep link for production builds.
More reliable than client-side window.location.href.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/app/api/desktop/callback/route.ts

@@ -0,0 +1,38 @@
+import { NextRequest, NextResponse } from 'next/server';
+
+/**
+ * Desktop Callback API
+ *
+ * Server-side redirect to deep link (multica://auth?...)
+ * This is more reliable than client-side window.location.href
+ *
+ * Reference: Cap/apps/web/app/api/desktop/[...route]/session.ts
+ */
+export async function GET(request: NextRequest) {
+  const { searchParams } = request.nextUrl;
+  const sid = searchParams.get('sid');
+  const user = searchParams.get('user');
+  const port = searchParams.get('port');
+  const platform = searchParams.get('platform') || 'desktop';
+
+  // Validate required params
+  if (!sid || !user) {
+    return NextResponse.json(
+      { error: 'Missing sid or user parameter' },
+      { status: 400 }
+    );
+  }
+
+  // Build callback URL
+  const params = new URLSearchParams({ sid, user });
+
+  if (platform === 'web' && port) {
+    // Dev mode: redirect to local server
+    const callbackUrl = `http://127.0.0.1:${port}/callback?${params}`;
+    return NextResponse.redirect(callbackUrl);
+  } else {
+    // Production mode: redirect to deep link
+    const deepLinkUrl = `multica://auth?${params}`;
+    return NextResponse.redirect(deepLinkUrl);
+  }
+}