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/utils/api-key-validation.ts

@@ -1,3 +1,4 @@
+import { base } from '$app/paths';
 import { error } from '@sveltejs/kit';
 import { browser } from '$app/environment';
 import { config } from '$lib/stores/settings.svelte';
@@ -22,7 +23,7 @@ export async function validateApiKey(fetch: typeof globalThis.fetch): Promise<vo
 			headers.Authorization = `Bearer ${apiKey}`;
 		}
 
-		const response = await fetch(`./props`, { headers });
+		const response = await fetch(`${base}/props`, { headers });
 
 		if (!response.ok) {
 			if (response.status === 401 || response.status === 403) {