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/stores/__tests__/auth.childmode.spec.ts

@@ -0,0 +1,33 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { isParentAuthenticated, loginUser } from '../auth'
+import { nextTick } from 'vue'
+
+// Helper to mock localStorage
+global.localStorage = {
+  store: {} as Record<string, string>,
+  getItem(key: string) {
+    return this.store[key] || null
+  },
+  setItem(key: string, value: string) {
+    this.store[key] = value
+  },
+  removeItem(key: string) {
+    delete this.store[key]
+  },
+  clear() {
+    this.store = {}
+  },
+} as any
+
+describe('auth store - child mode on login', () => {
+  beforeEach(() => {
+    isParentAuthenticated.value = true
+    localStorage.setItem('isParentAuthenticated', 'true')
+  })
+
+  it('should clear isParentAuthenticated and localStorage on loginUser()', async () => {
+    loginUser()
+    await nextTick() // flush Vue watcher
+    expect(isParentAuthenticated.value).toBe(false)
+  })
+})