feat: workspace frontend core (PR-3)

- Workspace types, API client, hooks (useWorkspaces, useWorkspaceActions)
- WorkspaceCard, WorkspaceCreateForm, StartToolModal components
- WorkspacesPage with list, create, sync, delete, start-tool flow
- Sidebar navigation: new 'Workspaces' entry
- Router: /workspaces route
- TypeScript + eslint clean

apps/web/src/hooks/use-workspaces.ts

@@ -0,0 +1,37 @@
+/** Hook for fetching workspaces. */
+
+import { useCallback, useEffect, useState } from "react";
+import { listWorkspaces } from "../api/workspaces";
+import type { Workspace } from "../types/workspace";
+
+export interface UseWorkspacesResult {
+  workspaces: Workspace[];
+  loading: boolean;
+  error: string | null;
+  refresh: () => Promise<void>;
+}
+
+export function useWorkspaces(projectId: string, repoId: string): UseWorkspacesResult {
+  const [workspaces, setWorkspaces] = useState<Workspace[]>([]);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+
+  const refresh = useCallback(async () => {
+    setLoading(true);
+    setError(null);
+    try {
+      const data = await listWorkspaces(projectId, repoId);
+      setWorkspaces(data);
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Failed to load workspaces");
+    } finally {
+      setLoading(false);
+    }
+  }, [projectId, repoId]);
+
+  useEffect(() => {
+    refresh();
+  }, [refresh]);
+
+  return { workspaces, loading, error, refresh };
+}