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/dashboard/messages/en.json

@@ -56,7 +56,14 @@
     "chooseCafe": "Choose a café",
     "chooseCafeSubtitle": "This number has access to several cafés. Pick one to continue.",
     "createNewCafe": "Create a new café",
-    "createNewCafeHint": "Want to start your own café with this number?"
+    "createNewCafeHint": "Want to start your own café with this number?",
+    "tabOtp": "One-time code",
+    "tabPassword": "Password",
+    "username": "Username",
+    "usernamePlaceholder": "Username",
+    "password": "Password",
+    "passwordPlaceholder": "Password",
+    "invalidCredentials": "Incorrect username or password."
   },
   "roles": {
     "owner": "Owner",
@@ -405,7 +412,8 @@
       "attendance": "Attendance",
       "leave": "Leave",
       "payroll": "Payroll",
-      "access": "Branch access"
+      "access": "Branch access",
+      "credentials": "Login credentials"
     },
     "myAttendance": "My attendance",
     "clockIn": "Clock in",
@@ -415,7 +423,22 @@
     "paid": "Paid",
     "markPaid": "Mark paid",
     "employeeCount": "Employees",
-    "monthYear": "Payroll month"
+    "monthYear": "Payroll month",
+    "credentials": {
+      "title": "Employee login credentials",
+      "subtitle": "Set a username and password for each employee so they can sign in without an OTP.",
+      "selectEmployee": "Select an employee first",
+      "username": "Username",
+      "usernamePlaceholder": "e.g. ali_barista",
+      "password": "Password (min 8 characters)",
+      "passwordPlaceholder": "New password",
+      "set": "Save credentials",
+      "remove": "Remove credentials",
+      "removeConfirm": "Are you sure? The employee will no longer be able to sign in with a password.",
+      "saved": "Credentials saved.",
+      "removed": "Credentials removed.",
+      "usernameTaken": "This username is already taken."
+    }
   },
   "reviews": {
     "title": "Customer reviews",