feat(FN-2030): add project markdown explorer to Documents view

- Add recursive markdown file discovery in file-service with excluded directories and sorted metadata output
- Expose GET /api/files/markdown and wire a frontend API client plus useProjectMarkdownFiles hook
- Redesign DocumentsView with project/task tabs, search, task grouping, and desktop/mobile file preview flows
- Add coverage for markdown scanning and DocumentsView behaviors, and update useDocuments tests for new API usage

packages/dashboard/app/hooks/useProjectMarkdownFiles.ts

@@ -0,0 +1,78 @@
+import { useState, useEffect, useRef, useCallback } from "react";
+import { fetchProjectMarkdownFiles, type MarkdownFileEntry } from "../api";
+
+export interface UseProjectMarkdownFilesResult {
+  files: MarkdownFileEntry[];
+  loading: boolean;
+  error: string | null;
+  refresh: () => Promise<void>;
+}
+
+/**
+ * Hook for fetching markdown files from the project workspace.
+ *
+ * Loading behavior matches useDocuments: loading is true only for the initial
+ * fetch, not for subsequent refreshes, to avoid content flicker.
+ */
+export function useProjectMarkdownFiles(projectId?: string): UseProjectMarkdownFilesResult {
+  const [files, setFiles] = useState<MarkdownFileEntry[]>([]);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+  const abortRef = useRef<AbortController | null>(null);
+  const initialLoadCompleteRef = useRef(false);
+
+  const refresh = useCallback(async () => {
+    if (abortRef.current) {
+      abortRef.current.abort();
+    }
+
+    const requestController = new AbortController();
+    abortRef.current = requestController;
+
+    const isInitial = !initialLoadCompleteRef.current;
+    if (isInitial) {
+      setLoading(true);
+    }
+
+    setError(null);
+
+    try {
+      const response = await fetchProjectMarkdownFiles(projectId);
+
+      if (requestController.signal.aborted) {
+        return;
+      }
+
+      setFiles(response.files);
+      initialLoadCompleteRef.current = true;
+    } catch (err: unknown) {
+      if (requestController.signal.aborted) {
+        return;
+      }
+
+      setError(err instanceof Error ? err.message : String(err));
+    } finally {
+      if (!requestController.signal.aborted && isInitial) {
+        setLoading(false);
+      }
+    }
+  }, [projectId]);
+
+  useEffect(() => {
+    initialLoadCompleteRef.current = false;
+    void refresh();
+
+    return () => {
+      if (abortRef.current) {
+        abortRef.current.abort();
+      }
+    };
+  }, [refresh]);
+
+  return {
+    files,
+    loading,
+    error,
+    refresh,
+  };
+}