[ADD] pos_payment_method_cashdro_rounding: migration to 18.0

Migrated from the 17.0 branch (4fbf328). APIs that no longer exist in 18.0:

* `order.selected_paymentline` -> `order.get_selected_paymentline()`
* `ErrorPopup` + `env.services.popup` -> `AlertDialog` + `env.services.dialog`
* `/** @odoo-module */`, implicit in `.esm.js` since 17.0

The rounding itself needed a different approach: `get_rounding_applied()` now
reads `taxTotals.order_rounding`, only computed over payments that are already
`is_done()`. The CashDro line is still `waiting` when the amount is requested,
so a literal port would always have added 0. `getDefaultAmountDueToPayIn()` is
the 18.0 equivalent: it applies `getRoundedRemaining()` honouring both
`cash_rounding` and `only_round_cash_method`, and is what core itself uses for
the default amount of a payment line.

Only `_cashdro_payment_url()` is patched now, instead of duplicating the whole
`cashdro_send_payment_request()`, so the request flow, the acknowledge, the
polling and the error handling stay in the base module. With no cash rounding
configured the patch delegates to `super` and changes nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pos_payment_method_cashdro_rounding/__init__.py

@@ -0,0 +1,2 @@
+# Copyright Criptomart
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).