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.

frontend/modules/directory/services/dto/client.ts

@@ -0,0 +1,19 @@
+export type Client = {
+    id: number
+    '@id'?: string
+    name: string
+    email: string | null
+    phone: string | null
+    street: string | null
+    city: string | null
+    postalCode: string | null
+}
+
+export type ClientWrite = {
+    name: string
+    email: string | null
+    phone: string | null
+    street: string | null
+    city: string | null
+    postalCode: string | null
+}