feat: add storage management features including user storage quota and usage tracking

- Updated database schema to include `storage_quota_bytes` and `plan` for users.
- Implemented storage usage calculations in `storageRepo` and added tests for storage functionality.
- Created `StorageManager` component to display and manage user storage.
- Enhanced user settings to show storage details and warnings for high usage.
- Added translations for new storage-related strings in English and Dutch.
- Updated styles for storage overview and details in CSS.

frontend/src/i18n/en.json

@@ -185,7 +185,22 @@
     "textSizeDesc": "Adjust the base font size for readability",
     "defaultFeedback": "Default Feedback",
     "defaultFeedbackLabel": "Feedback mode",
-    "defaultFeedbackDesc": "Choose when to see answer feedback during exams"
+    "defaultFeedbackDesc": "Choose when to see answer feedback during exams",
+    "storage": "Storage",
+    "storageUsage": "Storage Usage",
+    "storageOf": "of",
+    "storageExams": "Saved exams",
+    "storageResults": "Results",
+    "showDetails": "Show details",
+    "hideDetails": "Hide details",
+    "storageDetails": "Storage Details",
+    "noStorageItems": "No items using storage.",
+    "exam": "Exam",
+    "result": "Result",
+    "deleteExamConfirm": "Delete this exam? This cannot be undone.",
+    "deleteResultConfirm": "Delete this result? This cannot be undone.",
+    "storageWarningHigh": "You are using over 80% of your storage.",
+    "storageWarningCritical": "You are using over 95% of your storage. Delete some items to free up space."
   },
   "admin": {
     "administration": "Administration",