fix(auth) : use dedicated plainPassword field for password hashing

- Add non-persisted plainPassword field to User entity (write-only via API)
- Remove direct write access to password field
- Update UserPasswordHasherProcessor to hash from plainPassword
- Update frontend DTO and UserDrawer component

Ticket: T-009

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/services/dto/user-data.ts

@@ -12,7 +12,7 @@ export type UserData = {
 
 export type UserWrite = {
     username: string
-    password?: string
+    plainPassword?: string
     roles: string[]
     client?: string | null
     allowedProjects?: string[]