Auto-detect expense currency from the selected account

On create: if accountId is set, use that account's currency; else fall
back to the company's base currency (not hardcoded THB).
On update: if account changes, also update the expense currency to
match, so the ledger entry posts in the right currency.

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

src/routes/(app)/companies/[companyId]/expenses/+page.server.ts

@@ -7,6 +7,7 @@ import {
 	users,
 	categories,
 	companyAccounts,
+	companies,
 	invoices,
 	parties,
 	packages,
@@ -196,6 +197,29 @@ export const actions: Actions = {
 			.limit(1);
 		if (!proj) return fail(400, { action: 'submitExpense', error: 'Project not found' });
 
+		// Resolve currency: use the selected account's currency, else company base
+		let resolvedCurrency = 'THB';
+		if (accountId) {
+			const [acct] = await db
+				.select({ currency: companyAccounts.currency })
+				.from(companyAccounts)
+				.where(
+					and(
+						eq(companyAccounts.id, accountId),
+						eq(companyAccounts.companyId, params.companyId)
+					)
+				)
+				.limit(1);
+			if (acct) resolvedCurrency = acct.currency;
+		} else {
+			const [company] = await db
+				.select({ currency: companies.currency })
+				.from(companies)
+				.where(eq(companies.id, params.companyId))
+				.limit(1);
+			if (company) resolvedCurrency = company.currency;
+		}
+
 		await db.insert(expenses).values({
 			projectId: resolvedProjectId,
 			categoryId: categoryId || null,
@@ -205,7 +229,7 @@ export const actions: Actions = {
 			title,
 			description,
 			amount: Number(amountStr).toFixed(2),
-			currency: 'THB',
+			currency: resolvedCurrency,
 			expenseDate,
 			status: 'pending'
 		});

src/routes/(app)/companies/[companyId]/expenses/[expenseId]/+page.server.ts

@@ -213,6 +213,22 @@ export const actions: Actions = {
 		const amountChanged = newAmount !== existing.amount;
 		const accountChanged = (accountId ?? null) !== (existing.accountId ?? null);
 
+		// Resolve currency from the (possibly new) account
+		let resolvedCurrency: string | undefined = undefined;
+		if (accountId) {
+			const [acct] = await db
+				.select({ currency: companyAccounts.currency })
+				.from(companyAccounts)
+				.where(
+					and(
+						eq(companyAccounts.id, accountId),
+						eq(companyAccounts.companyId, params.companyId)
+					)
+				)
+				.limit(1);
+			if (acct) resolvedCurrency = acct.currency;
+		}
+
 		await db.transaction(async (tx) => {
 			await tx
 				.update(expenses)
@@ -226,6 +242,7 @@ export const actions: Actions = {
 					partyId,
 					accountId,
 					invoiceId,
+					...(resolvedCurrency ? { currency: resolvedCurrency } : {}),
 					updatedAt: new Date()
 				})
 				.where(eq(expenses.id, params.expenseId));