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/routes/+layout.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import '../app.css';
+	import { base } from '$app/paths';
 	import { page } from '$app/state';
 	import { untrack } from 'svelte';
 	import { ChatSidebar, DialogConversationTitleUpdate } from '$lib/components/app';
@@ -157,7 +158,7 @@
 				headers.Authorization = `Bearer ${apiKey.trim()}`;
 			}
 
-			fetch(`./props`, { headers })
+			fetch(`${base}/props`, { headers })
 				.then((response) => {
 					if (response.status === 401 || response.status === 403) {
 						window.location.reload();