perf(codebase): implement in-memory cache for file tree to persist state across tab navigations

vibn-frontend/components/project/gitea-file-tree.tsx

@@ -38,21 +38,50 @@ interface GiteaFileTreeProps {
   selectedPath?: string;
 }
 
+// ── In-memory cache to persist tree state across tab navigations ──
+const treeCache: Record<
+  string,
+  {
+    rootItems: TreeItem[];
+    childrenByPath: Record<string, TreeItem[]>;
+    expanded: Set<string>;
+  }
+> = {};
+
 export function GiteaFileTree({
   projectId,
   rootPath,
   onSelectFile,
   selectedPath,
 }: GiteaFileTreeProps) {
-  const [rootItems, setRootItems] = useState<TreeItem[] | null>(null);
+  const cacheKey = `${projectId}::${rootPath}`;
-  const [loading, setLoading] = useState(true);
+
+  const [rootItems, setRootItems] = useState<TreeItem[] | null>(() => {
+    return treeCache[cacheKey]?.rootItems ?? null;
+  });
+  const [loading, setLoading] = useState(() => !treeCache[cacheKey]);
   const [error, setError] = useState<string | null>(null);
-  const [expanded, setExpanded] = useState<Set<string>>(new Set());
+  const [expanded, setExpanded] = useState<Set<string>>(() => {
+    return treeCache[cacheKey]?.expanded ?? new Set();
+  });
   const [childrenByPath, setChildrenByPath] = useState<
     Record<string, TreeItem[]>
-  >({});
+  >(() => {
+    return treeCache[cacheKey]?.childrenByPath ?? {};
+  });
   const [loadingPaths, setLoadingPaths] = useState<Set<string>>(new Set());
 
+  // Keep cache synced with state updates
+  useEffect(() => {
+    if (rootItems) {
+      treeCache[cacheKey] = {
+        rootItems,
+        childrenByPath,
+        expanded,
+      };
+    }
+  }, [cacheKey, rootItems, childrenByPath, expanded]);
+
   const fetchPath = useCallback(
     async (path: string): Promise<TreeItem[]> => {
       const res = await fetch(
@@ -71,12 +100,12 @@ export function GiteaFileTree({
 
   // Load root whenever projectId or rootPath changes
   useEffect(() => {
+    // If we already loaded this from cache on mount, skip fetching again
+    if (treeCache[cacheKey]) return;
+
     let cancelled = false;
     setLoading(true);
     setError(null);
-    setRootItems(null);
-    setExpanded(new Set());
-    setChildrenByPath({});
 
     fetchPath(rootPath)
       .then(async (items) => {
@@ -119,7 +148,7 @@ export function GiteaFileTree({
     return () => {
       cancelled = true;
     };
-  }, [projectId, rootPath, fetchPath]);
+  }, [projectId, rootPath, fetchPath, cacheKey]);
 
   const toggleDir = useCallback(
     async (path: string) => {