feat: username/password authentication for admin and merchant panels

- Add PasswordHasher utility (PBKDF2/SHA-256, 100k iterations)
- Add Username + PasswordHash fields to Employee and SystemAdmin entities
- EF migration: AddPasswordLogin (nullable columns on both tables)
- Meezi.API: POST /api/auth/login (employee password login, CHOOSE_CAFE support)
- Meezi.API: PUT/DELETE /api/cafes/{id}/employees/{id}/credentials (Owner/Manager only)
- Meezi.Admin.API: POST /api/admin/auth/login + PUT /api/admin/auth/password
- Dashboard login page: OTP / Password tabs
- Admin login page: OTP / Password tabs
- HR screen: new Credentials tab for setting employee username/password
- PlatformDataSeeder: ensure system admin + integration settings in production
- Trial countdown banner: updated deadline to 1 Tir 1405 (Jun 22)
- i18n: fa/en/ar updated for all new UI strings

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/admin/messages/en.json

@@ -1086,7 +1086,14 @@
       "otp": "Verification code",
       "login": "Sign in",
       "error": "Login failed",
-      "devHint": "In development the OTP is logged by Admin API (DEV admin OTP)."
+      "devHint": "In development the OTP is logged by Admin API (DEV admin OTP).",
+      "tabOtp": "One-time code",
+      "tabPassword": "Password",
+      "username": "Username",
+      "usernamePlaceholder": "Username",
+      "password": "Password",
+      "passwordPlaceholder": "Password",
+      "invalidCredentials": "Incorrect username or password."
     },
     "dashboard": {
       "title": "Platform overview",