AuthZ layout title: reduce title spacing + shrink Assign role button #238

Part of #237.

Problem

The AuthZ page title band has excess empty space from padding/border around the h2
title, and the Assign role button is larger than desired.

Figma design

Scope

• Remove the extra space around the h2 page title (padding/border on the title band).
• Decrease the size of the Assign role button.

Files

• src/authz-module/components/AuthZTitle.tsx — title band uses py-5; h2.text-primary.
  The generic ActionButton here renders a Paragon Button at default size.
• src/authz-module/components/AddRoleButton.tsx — standalone Button, default size.

Notes

• Button size: pass size="sm" to the Paragon Button. Apply in both
  AddRoleButton.tsx and the ActionButton in AuthZTitle.tsx so all "assign role"
  entry points match. This issue owns the shared AddRoleButton change.
• Title spacing: reduce the py-5 band padding and the h2 surrounding spacing/border.
  Reconcile exact values with Figma (TBD).

Acceptance criteria

• Reduced whitespace around the title band, verified visually.
• Assign role button renders smaller (sm) across all placements.
• No behavior/layout regressions on desktop and mobile breakpoints.