server: (webui) add --webui-config (#18028)

* server/webui: add server-side WebUI config support

Add CLI arguments --webui-config (inline JSON) and --webui-config-file
(file path) to configure WebUI default settings from server side.

Backend changes:
- Parse JSON once in server_context::load_model() for performance
- Cache parsed config in webui_settings member (zero overhead on /props)
- Add proper error handling in router mode with try/catch
- Expose webui_settings in /props endpoint for both router and child modes

Frontend changes:
- Add 14 configurable WebUI settings via parameter sync
- Add tests for webui settings extraction
- Fix subpath support with base path in API calls

Addresses feedback from @ngxson and @ggerganov

* server: address review feedback from ngxson

* server: regenerate README with llama-gen-docs

tools/server/webui/src/lib/components/app/server/ServerErrorSplash.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+	import { base } from '$app/paths';
 	import { AlertTriangle, RefreshCw, Key, CheckCircle, XCircle } from '@lucide/svelte';
 	import { goto } from '$app/navigation';
 	import { Button } from '$lib/components/ui/button';
@@ -64,7 +65,7 @@
 			settingsStore.updateConfig('apiKey', apiKeyInput.trim());
 
 			// Test the API key by making a real request to the server
-			const response = await fetch('./props', {
+			const response = await fetch(`${base}/props`, {
 				headers: {
 					'Content-Type': 'application/json',
 					Authorization: `Bearer ${apiKeyInput.trim()}`