fix: add error interceptor redirection mechanism

Author: c4ffein

src/app/core/interceptors/error.interceptor.ts

@@ -1,7 +1,53 @@
-import { HttpInterceptorFn } from '@angular/common/http';
+import { HttpErrorResponse, HttpInterceptorFn } from '@angular/common/http';
+import { inject } from '@angular/core';
 import { throwError } from 'rxjs';
 import { catchError } from 'rxjs/operators';
+import { UserService } from '../auth/services/user.service';
 
+/**
+ * Global HTTP error interceptor.
+ *
+ * ## 401 Handling Strategy
+ *
+ * There are two layers of 401 handling:
+ *
+ * 1. GET /user endpoint (auth initialization):
+ *    - Handled by UserService.getCurrentUser() with 4XX vs 5XX logic
+ *    - This interceptor SKIPS /user to avoid double-handling
+ *
+ * 2. All OTHER endpoints (articles, comments, profiles, etc.):
+ *    - Handled HERE - any 401 means "token expired mid-session"
+ *    - We call purgeAuth() to logout immediately
+ *
+ * Both paths result in logout on 401, but /user needs special handling
+ * because it also distinguishes 5XX errors (server down → retry).
+ *
+ * ## Error Format
+ *
+ * Re-throws errors with { ...body, status } format so that:
+ * - Components can access err.errors for display (e.g., validation errors)
+ * - Auth logic can check err.status for error type decisions
+ * - Network errors get a user-friendly fallback message
+ */
 export const errorInterceptor: HttpInterceptorFn = (req, next) => {
-  return next(req).pipe(catchError(err => throwError(() => err.error)));
+  const userService = inject(UserService);
+
+  return next(req).pipe(
+    catchError((err: HttpErrorResponse) => {
+      // Global 401 handling for all endpoints EXCEPT /user
+      // (token expired mid-session → logout)
+      // /user is handled by UserService.getCurrentUser() with 4XX vs 5XX logic
+      if (err.status === 401 && !req.url.endsWith('/user')) {
+        userService.purgeAuth();
+      }
+
+      // Normalize error format: { errors: {...}, status: number }
+      // Provides fallback message for network errors (status 0) or missing body
+      const body = err.error && typeof err.error === 'object' && 'errors' in err.error
+        ? err.error
+        : { errors: { network: ['Unable to connect. Please check your internet connection.'] } };
+
+      return throwError(() => ({ ...body, status: err.status }));
+    }),
+  );
 };