more stuff to improve robustness

frontend/src/lib/assert.ts

@@ -0,0 +1,11 @@
+export function assert(condition: boolean, message: string): asserts condition {
+  if (!condition) {
+    const error = new Error(`Assertion failed: ${message}`);
+    if (import.meta.env.DEV) {
+      console.error('[Assertion]', message, error.stack);
+      throw error;
+    } else {
+      console.warn('[Assertion] (prod silenced):', message);
+    }
+  }
+}