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/services/parameter-sync.spec.ts

@@ -130,5 +130,19 @@ describe('ParameterSyncService', () => {
 			expect(result.max_tokens).toBe(-1);
 			expect(result.temperature).toBe(0.7);
 		});
+
+		it('should merge webui settings from props when provided', () => {
+			const result = ParameterSyncService.extractServerDefaults(null, {
+				pasteLongTextToFileLen: 0,
+				pdfAsImage: true,
+				renderUserContentAsMarkdown: false,
+				theme: 'dark'
+			});
+
+			expect(result.pasteLongTextToFileLen).toBe(0);
+			expect(result.pdfAsImage).toBe(true);
+			expect(result.renderUserContentAsMarkdown).toBe(false);
+			expect(result.theme).toBeUndefined();
+		});
 	});
 });