Link source expense/invoice from account transaction rows

Each transaction that was posted from an expense or invoice now shows
an 'Open expense →' or 'Open invoice →' link to the source record's
detail page. The sourceExpenseId/sourceInvoiceId fields were already
loaded — just needed the UI affordance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/routes/(app)/companies/[companyId]/accounts/[accountId]/+page.svelte

@@ -347,6 +347,22 @@
 										Counterparty: {txn.counterpartyName}
 									</div>
 								{/if}
+								{#if txn.sourceExpenseId}
+									<a
+										href={`/companies/${data.company.id}/expenses/${txn.sourceExpenseId}`}
+										class="mt-0.5 inline-block text-xs font-medium text-blue-600 hover:text-blue-700 dark:text-blue-400"
+									>
+										Open expense →
+									</a>
+								{/if}
+								{#if txn.sourceInvoiceId}
+									<a
+										href={`/companies/${data.company.id}/invoices/${txn.sourceInvoiceId}`}
+										class="mt-0.5 inline-block text-xs font-medium text-blue-600 hover:text-blue-700 dark:text-blue-400"
+									>
+										Open invoice →
+									</a>
+								{/if}
 								{#if txn.fxRate && txn.fxAmount}
 									<div class="text-xs text-amber-600 dark:text-amber-400">
 										FX: {txn.fxAmount} @ {Number(txn.fxRate).toFixed(4)}