feat: Implement account deletion (mark for removal) feature

- Added `marked_for_deletion` and `marked_for_deletion_at` fields to User model (Python and TypeScript) with serialization updates
- Created POST /api/user/mark-for-deletion endpoint with JWT auth, error handling, and SSE event trigger
- Blocked login and password reset for marked users; added new error codes ACCOUNT_MARKED_FOR_DELETION and ALREADY_MARKED
- Updated UserProfile.vue with "Delete My Account" button, confirmation modal (email input), loading state, success/error modals, and sign-out/redirect logic
- Synced error codes and model fields between backend and frontend
- Added and updated backend and frontend tests to cover all flows and edge cases
- All Acceptance Criteria from the spec are complete and verified

frontend/vue-app/src/assets/styles.css

@@ -50,6 +50,13 @@
   background: var(--btn-danger-hover);
 }
 
+.btn-danger:disabled {
+  background: var(--btn-secondary, #f3f3f3);
+  color: var(--btn-secondary-text, #666);
+  cursor: not-allowed;
+  opacity: 0.7;
+}
+
 /* Green button (e.g., Confirm) */
 .btn-green {
   background: var(--btn-green);