Ot_form: add reactive_fieldset

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: balat

src/widgets/ot_form.eliom

@@ -822,6 +822,12 @@ let%shared int_input ?(min = 0) ?(max = max_int) ?(size = 2) initial_value =
   in
   buttons, value
 
+(* -- Reactive fieldset ------------------------------------------- *)
+
+let%shared reactive_fieldset ?(a = []) ~disabled content =
+  let a = (a :> Html_types.fieldset_attrib attrib list) in
+  F.fieldset ~a:(R.filter_attrib (a_disabled ()) disabled :: a) content
+
 (* -- Date and time inputs ---------------------------------------- *)
 
 let%shared parse_date s =

src/widgets/ot_form.eliomi

@@ -320,6 +320,16 @@ val%shared validate_as_int : string -> (int option, unit) result
 val%shared none_input_value : string
 (** The string ["-"], used as placeholder for empty optional int inputs. *)
 
+(** {3 Reactive fieldset} *)
+
+val%shared reactive_fieldset :
+   ?a:[< Html_types.fieldset_attrib] attrib list
+  -> disabled:bool Eliom_shared.React.S.t
+  -> [< Html_types.fieldset_content] elt list
+  -> [> `Fieldset] elt
+(** A fieldset that can be reactively disabled.
+    When disabled, all form elements inside are disabled by the browser. *)
+
 (** {3 Date and time inputs} *)
 
 val%shared parse_date : string -> (int * int * int) option