feat: add flags to disable WebDAV and Admin UI in weed mini (#7971)

* feat: add flags to disable WebDAV and Admin UI in weed mini

- Add -webdav flag (default: true) to optionally disable WebDAV server
- Add -admin.ui flag (default: true) to optionally disable Admin UI only (server still runs)
- Conditionally skip WebDAV service startup based on flag
- Pass disableUI flag to SetupRoutes to skip UI route registration
- Admin server still runs for gRPC and API access when UI is disabled

Addresses issue from https://github.com/seaweedfs/seaweedfs/pull/7833#issuecomment-3711924150

* refactor: use positive enableUI parameter instead of disableUI across admin server and handlers

* docs: update mini welcome message to list enabled components

* chore: remove unused welcomeMessageTemplate constant

* docs: split S3 credential message into separate sb.WriteString calls

weed/admin/handlers/admin_handlers.go

@@ -49,7 +49,7 @@ func NewAdminHandlers(adminServer *dash.AdminServer) *AdminHandlers {
 }
 
 // SetupRoutes configures all the routes for the admin interface
-func (h *AdminHandlers) SetupRoutes(r *gin.Engine, authRequired bool, adminUser, adminPassword, readOnlyUser, readOnlyPassword string) {
+func (h *AdminHandlers) SetupRoutes(r *gin.Engine, authRequired bool, adminUser, adminPassword, readOnlyUser, readOnlyPassword string, enableUI bool) {
 	// Health check (no auth required)
 	r.GET("/health", h.HealthCheck)
 
@@ -61,6 +61,11 @@ func (h *AdminHandlers) SetupRoutes(r *gin.Engine, authRequired bool, adminUser,
 		c.Redirect(http.StatusMovedPermanently, "/static/favicon.ico")
 	})
 
+	// Skip UI routes if UI is not enabled
+	if !enableUI {
+		return
+	}
+
 	if authRequired {
 		// Authentication routes (no auth required)
 		r.GET("/login", h.authHandlers.ShowLogin)