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/components/auth/Signup.vue

@@ -107,8 +107,8 @@
           An account with <strong>{{ email }}</strong> already exists.
         </p>
         <div style="display: flex; gap: 2rem; justify-content: center">
-          <button @click="goToLogin" class="form-btn">Sign In</button>
-          <button @click="showEmailExistsModal = false" class="form-btn">Cancel</button>
+          <button @click="goToLogin" class="btn btn-primary">Sign In</button>
+          <button @click="handleCancelEmailExists" class="btn btn-secondary">Cancel</button>
         </div>
       </ModalDialog>
 
@@ -241,10 +241,15 @@ function goToLogin() {
   router.push({ name: 'Login' }).catch(() => (window.location.href = '/auth/login'))
 }
 
-// Clear password fields and close modal
+// Clear email and password fields and close modal
 function handleCancelEmailExists() {
+  email.value = ''
   password.value = ''
   confirmPassword.value = ''
+  emailTouched.value = false
+  passwordTouched.value = false
+  confirmTouched.value = false
+  signupError.value = ''
   showEmailExistsModal.value = false
 }