fix: preserved zero values in chat settings inputs and textareas by switching to nullish coalescing for field values and default placeholders (#16312)

tools/server/webui/src/lib/components/app/chat/ChatSettings/ChatSettingsFields.svelte

@@ -30,9 +30,9 @@
 
 			<Input
 				id={field.key}
-				value={String(localConfig[field.key] || '')}
+				value={String(localConfig[field.key] ?? '')}
 				onchange={(e) => onConfigChange(field.key, e.currentTarget.value)}
-				placeholder={`Default: ${SETTING_CONFIG_DEFAULT[field.key] || 'none'}`}
+				placeholder={`Default: ${SETTING_CONFIG_DEFAULT[field.key] ?? 'none'}`}
 				class={isMobile ? 'w-full' : 'max-w-md'}
 			/>
 			{#if field.help || SETTING_CONFIG_INFO[field.key]}
@@ -47,9 +47,9 @@
 
 			<Textarea
 				id={field.key}
-				value={String(localConfig[field.key] || '')}
+				value={String(localConfig[field.key] ?? '')}
 				onchange={(e) => onConfigChange(field.key, e.currentTarget.value)}
-				placeholder={`Default: ${SETTING_CONFIG_DEFAULT[field.key] || 'none'}`}
+				placeholder={`Default: ${SETTING_CONFIG_DEFAULT[field.key] ?? 'none'}`}
 				class={isMobile ? 'min-h-[100px] w-full' : 'min-h-[100px] max-w-2xl'}
 			/>
 			{#if field.help || SETTING_CONFIG_INFO[field.key]}