fix(frontend): Svelte 5 reactivity, each block keys, and path resolution

Add proper keys to #each blocks across 15+ components to fix reordering bugs. Replace new Date() with SvelteDate in reactive contexts. Use $derived for computed values (totalPages). Use resolve() from $app/paths for all internal navigation hrefs. Add ARIA labels and keyboard accessibility to NavBar mobile menu. Remove unused handleRetry from UserPaymentModal.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { authStore } from '$lib/stores/auth.svelte';
-	import { SvelteDate } from 'svelte/reactivity';
+	import { SvelteDate, SvelteSet } from 'svelte/reactivity';
 	import { toast } from 'svelte-sonner';
 	import { CalendarDate, getLocalTimeZone, type DateValue } from '@internationalized/date';
 
@@ -51,7 +51,7 @@
 	let workingHoursCache: Record<string, Record<string, any>> = {};
 	let availableHoursCache: Record<string, Record<string, any>> = {};
 	let loadingMonths: Record<string, boolean> = {};
-	let loadingMonthKeys: Set<string> = new Set();
+	let loadingMonthKeys = new SvelteSet<string>();
 	let initialLoadDone = $state(false);
 	let rescheduleAutoSelectDone = $state(false);
 
@@ -220,7 +220,7 @@
 
 		if (loadingMonths[monthKey]) return;
 		if (loadingMonthKeys.has(monthKey)) return;
-		loadingMonthKeys = new Set(loadingMonthKeys).add(monthKey);
+		loadingMonthKeys.add(monthKey);
 		loadingMonths[monthKey] = true;
 
 		hoursMonthGeneration++;
@@ -265,7 +265,7 @@
 			toast.error('Failed to load availability');
 		} finally {
 			delete loadingMonths[monthKey];
-			loadingMonthKeys = new Set([...loadingMonthKeys].filter((k) => k !== monthKey));
+			loadingMonthKeys.delete(monthKey);
 			loadingHours = false;
 		}
 	}
@@ -280,7 +280,7 @@
 		workingHoursCache = {};
 		availableHoursCache = {};
 		loadingMonths = {};
-		loadingMonthKeys = new Set();
+		loadingMonthKeys = new SvelteSet<string>();
 		rescheduleAutoSelectDone = false;
 		placeholder = new CalendarDate(
 			new SvelteDate().getFullYear(),