perf(sidebar): fix UI lag with 100+ databases (closes #201)

- Lower virtual scrolling threshold from 400 to 100 nodes so
  RecycleScroller activates earlier with many databases
- Memoize getConfig() with a computed Map to avoid O(n) linear
  search on every TreeItem computed property evaluation

src/composables/useFlatTree.ts

@@ -23,5 +23,5 @@ export function flattenTree(nodes: TreeNode[]): FlatTreeNode[] {
 }
 
 export function shouldVirtualizeFlatTree(count: number): boolean {
-  return count >= 400;
+  return count >= 100;
 }