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/EditBookingModal.svelte

@@ -2,6 +2,7 @@
 	import { apiFetch } from '$lib/utils/api';
 	import { SvelteDate } from 'svelte/reactivity';
 	import { toast } from 'svelte-sonner';
+	import { extractErrorMessage } from '$lib/utils/toast-safe';
 	import { formatDuration } from '$lib/utils/format';
 	import { formatUserName } from '$lib/utils/nameDisplay';
 	import { parseWallClockDate } from '$lib/utils/timeSlots';
@@ -138,7 +139,7 @@
 				notes = booking.notes || '';
 			} else {
 				const text = await response.text();
-				toast.error('Failed to load booking: ' + text);
+				toast.error('Failed to load booking: ' + extractErrorMessage(text));
 			}
 		} catch (err) {
 			console.error('Error fetching booking:', err);
@@ -331,7 +332,7 @@
 				onSaved?.();
 			} else {
 				const text = await response.text();
-				toast.error('Failed to update booking: ' + text);
+				toast.error('Failed to update booking: ' + extractErrorMessage(text));
 			}
 		} catch (err) {
 			console.error('Error saving booking:', err);
@@ -379,7 +380,7 @@
 				fetchBooking();
 			} else {
 				const text = await response.text();
-				toast.error('Failed to process refund: ' + text);
+				toast.error('Failed to process refund: ' + extractErrorMessage(text));
 			}
 		} catch (err) {
 			console.error('Error processing refund:', err);