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/payments/UserPaymentModal.svelte

@@ -354,11 +354,6 @@
 		makePayment('partial', Math.round(partialAmountNum * 100));
 	}
 
-	function handleRetry() {
-		status = 'idle';
-		error = null;
-	}
-
 	function handleClose() {
 		stopPolling();
 		onClose();