fix(auth): honor ?next= redirect through Google OAuth flow

The login page now encodes the ?next= param into the Google OAuth state
so the auth callback can redirect to the right destination (e.g.
/invite/{id}) after login, instead of always going to /issues.

apps/web/app/(auth)/login/page.tsx

@@ -37,6 +37,15 @@ function LoginPageContent() {
     router.push(ws ? nextUrl : "/onboarding");
   };
 
+  // Build Google OAuth state: encode platform + next URL so the callback
+  // can redirect to the right place after login.
+  const googleState = [
+    platform === "desktop" ? "platform:desktop" : "",
+    nextUrl !== "/issues" ? `next:${nextUrl}` : "",
+  ]
+    .filter(Boolean)
+    .join(",") || undefined;
+
   return (
     <LoginPage
       onSuccess={handleSuccess}
@@ -45,7 +54,7 @@ function LoginPageContent() {
           ? {
               clientId: googleClientId,
               redirectUri: `${window.location.origin}/auth/callback`,
-              state: platform === "desktop" ? "platform:desktop" : undefined,
+              state: googleState,
             }
           : undefined
       }

apps/web/app/auth/callback/page.tsx

@@ -39,8 +39,11 @@ function CallbackContent() {
       return;
     }
 
-    const state = searchParams.get("state");
-    const isDesktop = state === "platform:desktop";
+    const state = searchParams.get("state") || "";
+    const stateParts = state.split(",");
+    const isDesktop = stateParts.includes("platform:desktop");
+    const nextPart = stateParts.find((p) => p.startsWith("next:"));
+    const nextUrl = nextPart ? nextPart.slice(5) : null; // strip "next:" prefix
 
     const redirectUri = `${window.location.origin}/auth/callback`;
 
@@ -63,7 +66,9 @@ function CallbackContent() {
           qc.setQueryData(workspaceKeys.list(), wsList);
           const lastWsId = localStorage.getItem("multica_workspace_id");
           const ws = await hydrateWorkspace(wsList, lastWsId);
-          router.push(ws ? "/issues" : "/onboarding");
+          // Honor the ?next= redirect if present (e.g. /invite/{id})
+          const defaultDest = ws ? "/issues" : "/onboarding";
+          router.push(nextUrl || defaultDest);
         })
         .catch((err) => {
           setError(err instanceof Error ? err.message : "Login failed");