Lower adaptive image threshold from 300px to 200px effective width

Keeps full-resolution images visible longer before swapping to
thumbnails, only downscaling when nodes are quite small on screen.

https://claude.ai/code/session_01MvD1n4QeXutgwUpKJuDGHa

src/hooks/useAdaptiveImageSrc.ts

@@ -9,7 +9,7 @@ import {
   removePending,
 } from "@/store/thumbnailCache";
 
-const EFFECTIVE_WIDTH_THRESHOLD = 300;
+const EFFECTIVE_WIDTH_THRESHOLD = 200;
 const DEFAULT_NODE_WIDTH = 350;
 
 /**