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/account/UserBookingModal.svelte

@@ -6,6 +6,7 @@
 	import { ensureBusinessInfo, getBusinessInfo } from '$lib/stores/businessInfo.svelte';
 	import { SvelteDate } from 'svelte/reactivity';
 	import { toast } from 'svelte-sonner';
+	import { extractErrorMessage } from '$lib/utils/toast-safe';
 	import * as Modal from '$lib/components/ui/dialog';
 	import { Button } from '$lib/components/ui/button';
 	import { Input } from '$lib/components/ui/input';
@@ -231,7 +232,7 @@
 				pendingEditRequest = editData.edit_request || null;
 			} else {
 				const text = await bookingResp.text();
-				toast.error('Failed to load booking: ' + text);
+				toast.error('Failed to load booking: ' + extractErrorMessage(text));
 				open = false;
 			}
 		} catch (err) {
@@ -361,7 +362,7 @@ ${hasVAT ? `<p class="warning">VAT is included at ${biz?.default_vat_rate ?? 20}
 				open = false;
 			} else {
 				const text = await response.text();
-				toast.error('Failed to cancel: ' + text);
+				toast.error('Failed to cancel: ' + extractErrorMessage(text));
 			}
 		} catch {
 			toast.error('Network error');