feat: buy crypto with fiat

diff --git a/common/protob/messages-common.proto b/common/protob/messages-common.proto
index 1777b3c44..a3a2be1b4 100644
--- a/common/protob/messages-common.proto
+++ b/common/protob/messages-common.proto
@@ -185,7 +185,8 @@ message PaymentRequest {
     required string recipient_name = 2;    // merchant's name
     repeated PaymentRequestMemo memos = 3; // any memos that were signed as part of the request
     reserved 4;                            // this existed briefly. obsoleted by this change: https://github.com/satoshilabs/slips/commit/08d36aa61722275a21617ac6a713e31ec23fdec4
-    optional bytes amount = 6;             // the sum of the external output amounts requested, required for non-CoinJoin, encoded in little endian on either 8 or 32 bytes
+    optional bytes amount = 6;             // the sum of the external output amounts requested, encoded in little endian on either 8 or 32 bytes
+                                           // required for non-CoinJoin transactions; do not set this for fiat amounts (BUY crypto with fiat)
     required bytes signature = 5;          // the trusted party's signature of the paymentRequestDigest
 
     message PaymentRequestMemo {
diff --git a/core/src/apps/misc/payment_notification.py b/core/src/apps/misc/payment_notification.py
index f7cfed7fd..f48ea83fd 100644
--- a/core/src/apps/misc/payment_notification.py
+++ b/core/src/apps/misc/payment_notification.py
@@ -3,23 +3,65 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from trezor.messages import PaymentNotification, Success
 
-    from apps.common.keychain import Keychain
+from apps.common.paths import address_n_to_str
 
-from apps.common.keychain import with_slip44_keychain
+# this module implements SLIP-0024 payment requests for crypto purchases using fiat
 
 
-@with_slip44_keychain(slip44_id=0, slip21_namespaces=[[b"SLIP-0024"]])
-async def payment_notification(msg: PaymentNotification, keychain: Keychain) -> Success:
+async def payment_notification(msg: PaymentNotification) -> Success:
     from trezor.messages import Success
+    from trezor.ui.layouts import confirm_payment_request
     from trezor.wire import DataError
 
+    from apps.common.keychain import get_keychain
     from apps.common.payment_request import PaymentRequestVerifier
 
     if msg.payment_req is None:
         raise DataError("Missing payment request.")
 
-    PaymentRequestVerifier(msg.payment_req, 0, keychain).verify()
+    if msg.payment_req.amount is not None:
+        raise DataError("Payment request amount must be missing")
 
-    # TODO Show payment request memos.
+    slip21_keychain = await get_keychain("", [], [[b"SLIP-0024"]])
+    PaymentRequestVerifier(msg.payment_req, 0, slip21_keychain).verify()
+
+    verified_payment_request = msg.payment_req
+
+    texts: list[tuple[str | None, str]] = []
+    trades: list[tuple[str | None, str, str, str | None, str | None]] = []
+    for memo in verified_payment_request.memos:
+        # Note: we do not process RefundMemo here:
+        # if the swap fails, the fiat amount just remains in your custodial account, it does not get refunded anywhere
+        if memo.text_memo is not None:
+            texts.append((None, memo.text_memo.text))
+        elif memo.text_details_memo is not None:
+            texts.append((memo.text_details_memo.title, memo.text_details_memo.text))
+        elif memo.coin_purchase_memo:
+            coin_purchase_account_path = address_n_to_str(
+                memo.coin_purchase_memo.address_n
+            )
+            trades.append(
+                (
+                    None,  # if we later decide to somehow pass the fiat amount (and currency!) as part of the payment request in a more structured fashion,
+                    # we should include it here so it gets shown on the trade screen, but for now we just have the fiat amount ad-hoc as part of a text memo.
+                    f"+\u00a0{memo.coin_purchase_memo.amount}",  # amount of crypto purchased
+                    memo.coin_purchase_memo.address,
+                    None,
+                    coin_purchase_account_path,
+                )
+            )
+        else:
+            raise DataError("Unrecognized memo type in payment request memo.")
+
+    await confirm_payment_request(
+        recipient_name=verified_payment_request.recipient_name,
+        recipient_address=None,  # no address for the fiat being spent
+        texts=texts,
+        refunds=[],
+        trades=trades,
+        account_items=[],
+        transaction_fee=None,
+        fee_info_items=None,
+    )
 
     return Success()