feat: reorganize admin settings with tabbed interface and documentation

- Reorganized admin settings into tabbed interface (General, Security, Payment Methods)
- Vertical tabs on desktop, horizontal scrollable on mobile
- Moved Payment Methods from separate menu to Settings tab
- Fixed admin profile reuse and dashboard blocking
- Fixed maintenance mode guard to use AppConfig model
- Added admin auto-redirect after login (admins → /admin, users → /)
- Reorganized documentation into docs/ folder structure
- Created comprehensive README and documentation index
- Added PWA and Web Push notifications to to-do list

apps/api/src/common/guards/maintenance.guard.ts

@@ -0,0 +1,72 @@
+import {
+  Injectable,
+  CanActivate,
+  ExecutionContext,
+  ServiceUnavailableException,
+} from '@nestjs/common';
+import { Reflector } from '@nestjs/core';
+import { PrismaService } from '../../prisma/prisma.service';
+import { JwtService } from '@nestjs/jwt';
+
+@Injectable()
+export class MaintenanceGuard implements CanActivate {
+  constructor(
+    private reflector: Reflector,
+    private prisma: PrismaService,
+    private jwtService: JwtService,
+  ) {}
+
+  async canActivate(context: ExecutionContext): Promise<boolean> {
+    // Check if route is exempt from maintenance mode (auth, health, admin routes)
+    const isExempt = this.reflector.get<boolean>('skipMaintenance', context.getHandler());
+    const isControllerExempt = this.reflector.get<boolean>('skipMaintenance', context.getClass());
+    
+    if (isExempt || isControllerExempt) {
+      return true;
+    }
+
+    // Get maintenance mode status from config
+    const maintenanceConfig = await this.prisma.appConfig.findUnique({
+      where: { key: 'maintenance_mode' },
+    });
+
+    const isMaintenanceMode = maintenanceConfig?.value === 'true';
+
+    if (!isMaintenanceMode) {
+      return true;
+    }
+
+    // Try to extract user from JWT token (if exists)
+    const request = context.switchToHttp().getRequest();
+    const authHeader = request.headers.authorization;
+    
+    if (authHeader && authHeader.startsWith('Bearer ')) {
+      try {
+        const token = authHeader.substring(7);
+        const payload = this.jwtService.verify(token);
+        
+        // If user is admin, allow access
+        if (payload.role === 'admin') {
+          return true;
+        }
+      } catch (error) {
+        // Invalid token, continue to block
+      }
+    }
+
+    // Get maintenance message
+    const messageConfig = await this.prisma.appConfig.findUnique({
+      where: { key: 'maintenance_message' },
+    });
+
+    const message =
+      messageConfig?.value ||
+      'System is under maintenance. Please try again later.';
+
+    throw new ServiceUnavailableException({
+      statusCode: 503,
+      message: message,
+      maintenanceMode: true,
+    });
+  }
+}