feat: standardize shared UI primitives across frontend

frontend/src/api/client.ts

@@ -11,8 +11,8 @@ export const apiClient = axios.create({
   },
 })
 
-// Global error handler - shows toast for common API errors
-// Pages can still catch errors explicitly if they need custom handling
+// Global error handler for shared cases only.
+// By convention, 4xx errors are handled at the page/component level.
 function handleGlobalError(error: AxiosError) {
   // Network error (no response from server)
   if (!error.response) {
@@ -43,9 +43,8 @@ function handleGlobalError(error: AxiosError) {
     return
   }
 
-  // Client errors (4xx) — don't toast globally.
-  // Pages handle their own 4xx errors (permission checks, validation, not-found)
-  // and many are caught silently. Global toasts here cause noisy duplicates.
+  // Client errors (4xx) remain page-owned to avoid duplicate/noisy toasts.
+  // Global handling only covers 401/429/5xx.
   if (status >= 400 && status < 500) {
     return
   }