feat(FN-1203): extract App orchestration into focused hooks

- Extract modal manager, app settings, deep-link handling, favorites, and auth onboarding into dedicated hooks
- Rewire App.tsx to consume the new hooks and reduce component-level state/effect complexity
- Preserve favorite-toggle error toast behavior in the updated favorites wiring
- Add hook-level test coverage for modal manager, app settings, deep links, favorites, and auth onboarding flows

packages/dashboard/app/hooks/useDeepLink.ts

@@ -0,0 +1,105 @@
+import { useCallback, useEffect, useRef } from "react";
+import type { TaskDetail } from "@fusion/core";
+import { fetchTaskDetail, type ProjectInfo } from "../api";
+import type { ToastType } from "./useToast";
+
+interface UseDeepLinkOptions {
+  projectId?: string;
+  projects: ProjectInfo[];
+  projectsLoading: boolean;
+  currentProject: ProjectInfo | null;
+  setCurrentProject: (project: ProjectInfo) => void;
+  addToast: (message: string, type?: ToastType) => void;
+  openTaskDetail: (task: TaskDetail) => void;
+  closeTaskDetail: () => void;
+}
+
+export interface UseDeepLinkResult {
+  /**
+   * Call when the task detail modal closes.
+   * Cleans ?task=... from URL if the modal was opened via deep-link.
+   */
+  handleDetailClose: () => void;
+}
+
+/**
+ * Handles task deep-link behavior (?project=...&task=...).
+ */
+export function useDeepLink(options: UseDeepLinkOptions): UseDeepLinkResult {
+  const {
+    projectId,
+    projects,
+    projectsLoading,
+    currentProject,
+    setCurrentProject,
+    addToast,
+    openTaskDetail,
+    closeTaskDetail,
+  } = options;
+
+  // Prevent duplicate fetches when project switching causes the effect to re-run.
+  const deepLinkFetchedRef = useRef(false);
+
+  // Track whether the currently open detail modal came from a deep-link.
+  const deepLinkTaskIdRef = useRef<string | null>(null);
+
+  useEffect(() => {
+    const params = new URLSearchParams(window.location.search);
+    const projectParam = params.get("project");
+    const taskId = params.get("task");
+
+    if (!taskId) return;
+    if (projectsLoading) return;
+
+    if (projectParam) {
+      const matchingProject = projects.find((project) => project.id === projectParam);
+      if (!matchingProject) {
+        addToast(`Project '${projectParam}' not found`, "error");
+        return;
+      }
+
+      if (currentProject?.id !== matchingProject.id) {
+        setCurrentProject(matchingProject);
+      }
+    }
+
+    if (deepLinkFetchedRef.current) return;
+    deepLinkFetchedRef.current = true;
+
+    const taskProjectId = projectParam ?? projectId;
+    fetchTaskDetail(taskId, taskProjectId)
+      .then((detail) => {
+        openTaskDetail(detail);
+        deepLinkTaskIdRef.current = taskId;
+      })
+      .catch(() => {
+        addToast(`Task ${taskId} not found`, "error");
+      });
+  }, [
+    projectId,
+    projects,
+    projectsLoading,
+    currentProject,
+    setCurrentProject,
+    addToast,
+    openTaskDetail,
+  ]);
+
+  const handleDetailClose = useCallback(() => {
+    if (deepLinkTaskIdRef.current) {
+      const params = new URLSearchParams(window.location.search);
+      params.delete("task");
+      const query = params.toString();
+      window.history.replaceState(
+        null,
+        "",
+        query ? `${window.location.pathname}?${query}` : window.location.pathname,
+      );
+      deepLinkTaskIdRef.current = null;
+    }
+
+    closeTaskDetail();
+  }, [closeTaskDetail]);
+
+  return { handleDetailClose };
+}