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

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