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/today/TodayCalendar.svelte

@@ -920,7 +920,7 @@
 							checkOverlappingBookings();
 						}}
 					>
-						{#each availableStartOptions as opt}
+						{#each availableStartOptions as opt (opt.totalMin)}
 							<option value={`${opt.hour}:${opt.minute}:${opt.period}`}>{opt.label}</option>
 						{/each}
 					</select>
@@ -948,7 +948,7 @@
 							checkOverlappingBookings();
 						}}
 					>
-						{#each availableEndOptions as opt}
+						{#each availableEndOptions as opt (opt.totalMin)}
 							<option value={`${opt.hour}:${opt.minute}:${opt.period}`}>{opt.label}</option>
 						{/each}
 					</select>
@@ -1047,7 +1047,7 @@
 </Modal.Root>
 
 <AlertDialog.Root bind:open={showDeleteAlert}>
-	<AlertDialog.Content class="z-[60]">
+	<AlertDialog.Content class="z-60">
 		<AlertDialog.Header>
 			<AlertDialog.Title>Delete time blocker?</AlertDialog.Title>
 			<AlertDialog.Description