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/stores/server.svelte.ts

@@ -40,6 +40,10 @@ class ServerStore {
 		return this.props?.default_generation_settings?.n_ctx ?? null;
 	}
 
+	get webuiSettings(): Record<string, string | number | boolean> | undefined {
+		return this.props?.webui_settings;
+	}
+
 	get isRouterMode(): boolean {
 		return this.role === ServerRole.ROUTER;
 	}