Add HST/tax support and payment reporting with HST aggregation

Studio Settings gains a default HST rate; the rate is frozen onto each
payment at booking and computed against the pre-tax subtotal, with the
total billed as subtotal + tax. The rate is overridable per booking on
My Lessons while unpaid (recomputing the tax amount), comped
registrations are never taxed, and receipts break out subtotal/HST/total.

Builds the payments report (roadmap #8) from us_payments: a monthly
per-instructor view with subtotal, HST collected, and grand-total
aggregation, plus a nonce-protected CSV export via admin-post. Studio
admins see all instructors and can filter; instructors are scoped to
their own rows. The Payment Report menu is gated on export_payments so
instructors (who lack manage_billing) can reach it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Payment/ReceiptMailer.php

@@ -20,13 +20,26 @@ class ReceiptMailer {
 			(string) $payment->receiptNumber
 		);
 
-		$body = sprintf(
-			/* translators: 1: amount, 2: currency, 3: receipt number */
-			__( "Thank you. We have recorded your payment of %1\$s %2\$s.\n\nReceipt: %3\$s", 'unsupervised-schedular' ),
-			number_format( $payment->amount, 2 ),
-			$payment->currency,
-			(string) $payment->receiptNumber
-		);
+		if ( $payment->taxAmount > 0 ) {
+			$body = sprintf(
+				/* translators: 1: currency, 2: subtotal, 3: HST rate, 4: HST amount, 5: total, 6: receipt number */
+				__( "Thank you. We have recorded your payment.\n\nSubtotal: %1\$s %2\$s\nHST (%3\$s%%): %1\$s %4\$s\nTotal: %1\$s %5\$s\n\nReceipt: %6\$s", 'unsupervised-schedular' ),
+				$payment->currency,
+				number_format( $payment->amount, 2 ),
+				number_format( $payment->taxRate, 2 ),
+				number_format( $payment->taxAmount, 2 ),
+				number_format( $payment->total(), 2 ),
+				(string) $payment->receiptNumber
+			);
+		} else {
+			$body = sprintf(
+				/* translators: 1: amount, 2: currency, 3: receipt number */
+				__( "Thank you. We have recorded your payment of %1\$s %2\$s.\n\nReceipt: %3\$s", 'unsupervised-schedular' ),
+				number_format( $payment->total(), 2 ),
+				$payment->currency,
+				(string) $payment->receiptNumber
+			);
+		}
 
 		return (bool) wp_mail( $student->user_email, $subject, $body );
 	}