fix: update border color for nodes and sidebar to align with design system

frontend/src/styles.css

@@ -164,12 +164,6 @@ pre {
     scrollbar-color: hsl(var(--muted-foreground) / 0.35) transparent;
 }
 
-*,
-::before,
-::after {
-    border-color: unset;
-}
-
 .dark * {
     scrollbar-color: hsl(var(--muted-foreground) / 0.4) transparent;
 }
@@ -196,6 +190,14 @@ pre {
     background: transparent;
 }
 
+/* Override Tailwind preflight: use theme border color instead of hardcoded #e5e7eb
+   so nodes and sidebar borders respect the design system. */
+*,
+::before,
+::after {
+    border-color: hsl(var(--border));
+}
+
 .dark *::-webkit-scrollbar-thumb {
     background: hsl(var(--muted-foreground) / 0.4);
 }