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

common/common.h

@@ -484,8 +484,11 @@ struct common_params {
 
     std::map<std::string, std::string> default_template_kwargs;
 
+    // webui configs
+    bool webui = true;
+    std::string webui_config_json;
+
     // "advanced" endpoints are disabled by default for better security
-    bool webui            = true;
     bool endpoint_slots   = true;
     bool endpoint_props   = false; // only control POST requests, not GET
     bool endpoint_metrics = false;