Add email notification preference for task completion

- Add emailTaskCompleted field to NotificationPreference model
- Add email preference toggle to notification settings UI (iOS & Android)
- Add localized strings for email notifications section
- Update ViewModel to support email preference updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

iosApp/iosApp/Localizable.xcstrings

@@ -17844,6 +17844,17 @@
         }
       }
     },
+    "profile_email_notifications" : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Email Notifications"
+          }
+        }
+      }
+    },
     "profile_email_required_unique" : {
       "extractionState" : "manual",
       "localizations" : {
@@ -17909,6 +17920,28 @@
         }
       }
     },
+    "profile_email_task_completed" : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Task Completed Email"
+          }
+        }
+      }
+    },
+    "profile_email_task_completed_description" : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Receive email when a task is completed"
+          }
+        }
+      }
+    },
     "profile_first_name" : {
       "extractionState" : "manual",
       "localizations" : {