Make configui defaults first-class

Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>

Author: Copilot

frontend/app/configui/configvalidation.test.ts

@@ -3,6 +3,8 @@
 
 import { describe, expect, it } from "vitest";
 import {
+    getEffectiveConfigValue,
+    isConfigValueOverridden,
     normalizeConfigStringInput,
     validateConfigNumberInput,
     validateConfigStringInput,
@@ -32,4 +34,12 @@ describe("configvalidation", () => {
             value: 256,
         });
     });
+
+    it("distinguishes overridden values from inherited defaults", () => {
+        expect(isConfigValueOverridden(undefined)).toBe(false);
+        expect(isConfigValueOverridden(false)).toBe(true);
+        expect(isConfigValueOverridden(0.95)).toBe(true);
+        expect(getEffectiveConfigValue(undefined, 0.95)).toBe(0.95);
+        expect(getEffectiveConfigValue(0.95, 0.9)).toBe(0.95);
+    });
 });

frontend/app/configui/configvalidation.ts

@@ -15,6 +15,17 @@ export type ConfigNumberValidationOptions = {
     integer?: boolean;
 };
 
+export function isConfigValueOverridden<T>(value: T | undefined): boolean {
+    return value != null;
+}
+
+export function getEffectiveConfigValue<T>(value: T | undefined, defaultValue: T): T {
+    if (value != null) {
+        return value;
+    }
+    return defaultValue;
+}
+
 export function normalizeConfigStringInput(value: string, options?: ConfigStringValidationOptions): string {
     if (options?.trim === false) {
         return value;