fix(magasins): make the store form modal scrollable

The store contact and SMTP sections made the form taller than the viewport
and the dialog had no scroll, cutting off the bottom fields and buttons.
The dialog now caps at 85vh with internal scrolling, slightly widened so
the two-column SMTP fields breathe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FHdE9nEh8zHhQNGpCgrtYe

client/src/pages/Groups.tsx

@@ -505,7 +505,7 @@ export default function Groups() {
         setSelectedGroup(null);
         setFormData({ ...EMPTY_GROUP_FORM, invoiceColumnName: "Ref Facture" });
       }}>
-        <DialogContent className="sm:max-w-md" aria-describedby="group-modal-description">
+        <DialogContent className="sm:max-w-lg max-h-[85vh] overflow-y-auto" aria-describedby="group-modal-description">
           <DialogHeader>
             <DialogTitle>
               {selectedGroup ? 'Modifier' : 'Nouveau'} Groupe