fix: sanitize API error text display and add time_blockers tests

Add extractErrorMessage helper for JSON error body parsing and apply sanitizeText across all toast displays. Add time_blockers test coverage for new holiday placeholder cleanup and overlapping scenarios.

frontend/src/lib/components/admin/BookingCreateModal.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { apiFetch } from '$lib/utils/api';
 	import { toast } from 'svelte-sonner';
+	import { extractErrorMessage } from '$lib/utils/toast-safe';
 	import { SvelteDate, SvelteSet, SvelteURLSearchParams } from 'svelte/reactivity';
 	import { CalendarDate, getLocalTimeZone, type DateValue } from '@internationalized/date';
 	import { isValidUKPhone, toE164UK } from '$lib/utils/phone';
@@ -718,7 +719,7 @@
 				return false;
 			} else {
 				const errorText = await response.text();
-				toast.error(`Failed to reserve slot: ${errorText}`);
+				toast.error(`Failed to reserve slot: ${extractErrorMessage(errorText)}`);
 				return false;
 			}
 		} catch {
@@ -855,7 +856,7 @@
 				toast.success('Custom service created and added');
 			} else {
 				const err = await response.text();
-				toast.error(`Failed: ${err}`);
+				toast.error(`Failed: ${extractErrorMessage(err)}`);
 			}
 		} catch {
 			toast.error('Network error');
@@ -1014,7 +1015,7 @@
 				onBookingCreated?.();
 			} else {
 				const errorText = await res.text();
-				toast.error(`Failed to create booking: ${errorText}`);
+				toast.error(`Failed to create booking: ${extractErrorMessage(errorText)}`);
 			}
 		} catch {
 			toast.error('An error occurred while creating booking');