feat: add mindmap components from jarvis frontend

- Copied mindmap visualization components (ReactFlow-based interactive graph)
- Added MindmapViewer, ReactFlowEditor, MermaidViewer
- Included all node types: Concept, Task, Idea, Project
- Added controls: NodeCreateModal, ExportButton
- Created mindmap route at /mindmap
- Added useGraphData hook for knowledge graph API
- Copied auth-client and api utilities (dependencies)

Note: Requires better-auth packages to be installed for full compilation

apps/web/src/app/mindmap/page.tsx

@@ -0,0 +1,33 @@
+import { Metadata } from 'next';
+import { MindmapViewer } from '@/components/mindmap';
+
+export const metadata: Metadata = {
+  title: 'Mindmap | Mosaic',
+  description: 'Knowledge graph visualization',
+};
+
+/**
+ * Mindmap page - Interactive knowledge graph visualization
+ *
+ * Displays an interactive mindmap/knowledge graph using ReactFlow,
+ * with support for multiple node types (concepts, tasks, ideas, projects)
+ * and relationship visualization.
+ */
+export default function MindmapPage() {
+  return (
+    <div className="flex flex-col h-screen">
+      <header className="border-b border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-800 px-6 py-4">
+        <h1 className="text-2xl font-bold text-gray-900 dark:text-gray-100">
+          Knowledge Graph
+        </h1>
+        <p className="text-sm text-gray-600 dark:text-gray-400 mt-1">
+          Explore and manage your knowledge network
+        </p>
+      </header>
+
+      <main className="flex-1 overflow-hidden">
+        <MindmapViewer />
+      </main>
+    </div>
+  );
+}