feat(directory) : add Clients/Prospects repertoire front layer

LST-58 (2.4) front. Completes the Directory module.

- New frontend/modules/directory/ layer (auto-detected): /directory page with
  Clients and Prospects tabs.
- Client front moved into the layer (clients service + client DTO +
  ClientDrawer). New prospects service, prospect DTO and ProspectDrawer (with
  a "Convert to client" action calling POST /prospects/{id}/convert).
- Consumers repointed to ~/modules/directory/... (admin client tab, PM project
  drawer + project pages + project DTO, time-tracking page + export drawer).
- Sidebar admin item /directory gated by the directory module; /directory
  protected by the admin middleware. i18n keys added (directory.*, prospects.*).

nuxt build passes; routes preserved.

Adds the 2.4 plan doc.

config/sidebar.php

@@ -31,6 +31,7 @@ return [
         'roles' => ['ROLE_ADMIN'],
         'items' => [
             ['label' => 'sidebar.admin.teamAbsences', 'to' => '/team-absences', 'icon' => 'mdi:calendar-account-outline', 'module' => 'absence'],
+            ['label' => 'sidebar.admin.directory', 'to' => '/directory', 'icon' => 'mdi:contact-multiple-outline', 'module' => 'directory'],
             ['label' => 'sidebar.admin.administration', 'to' => '/admin', 'icon' => 'mdi:cog-outline', 'permission' => 'core.users.view'],
         ],
     ],