Security hardening and edge case fixes across frontend

Security Improvements:
- Add input sanitization utilities (XSS, SQL injection prevention)
- Add token validation with JWT structure verification
- Add secure form validators with pattern enforcement
- Implement proper token storage with encryption support

Service Hardening:
- Add timeout (30s) and retry logic (3 attempts) to all API calls
- Add UUID validation for all ID parameters
- Add null/undefined checks with defensive defaults
- Proper error propagation with typed error handling

Component Fixes:
- Fix memory leaks with takeUntilDestroyed pattern
- Remove mock data fallbacks in error handlers
- Add proper loading/error state management
- Add form field length limits and validation

Files affected: 51 (6000+ lines added for security)

frontend/src/app/features/admin/user-list/user-list.component.ts

@@ -1,9 +1,10 @@
-import { Component, OnInit } from '@angular/core';
+import { Component, OnInit, signal } from '@angular/core';
 import { CommonModule } from '@angular/common';
 import { MatTableModule } from '@angular/material/table';
 import { MatChipsModule } from '@angular/material/chips';
 import { MatCardModule } from '@angular/material/card';
 import { ApiService } from '../../../core/services/api.service';
+import { NotificationService } from '../../../core/services/notification.service';
 
 @Component({
   selector: 'app-user-list',
@@ -52,14 +53,20 @@ import { ApiService } from '../../../core/services/api.service';
 export class UserListComponent implements OnInit {
   users: any[] = [];
   displayedColumns = ['name', 'email', 'role', 'wallet'];
+  readonly loading = signal(false);
 
-  constructor(private api: ApiService) {}
+  constructor(private api: ApiService, private notification: NotificationService) {}
 
   async ngOnInit() {
+    this.loading.set(true);
     try {
       this.users = await this.api.get<any[]>('/admin/users').toPromise() || [];
     } catch (error) {
-      console.error('Failed to load users', error);
+      this.loading.set(false);
+      this.notification.error('Failed to load users. Please try again.');
+      console.error('Error:', error);
+    } finally {
+      this.loading.set(false);
     }
   }
 }