feat(desktop): add GoodDollar deposit option

[kotevcode]

Adds a "Deposit with GoodDollar" option under More options in the Add Credits chooser, below Meridian.

• Clicking it opens GoodDollar's hosted AI-credits page (https://aicredits.gooddollar.org/) in the browser with buyerAddress and operatorSignature query params.
• The main process reads the buyer's current operator nonce from the Deposits contract and signs the EIP-712 SetOperator message authorizing GoodDollar's operator wallet (0x192288D921045aa96903e5286E116960e5fb4607), so their side can submit setOperator() and run the $G deposit without the buyer paying gas.
• URL and operator address are overridable via payments.gooddollar.{url,operator} in the desktop config.
• The row uses GoodDollar's official mark with a Celo network badge and a "Pay with $G" caption; the button disables while the signature is being prepared.

Note for the GoodDollar side: the signature only covers the initial setOperator() — if a buyer already has an operator set, the call reverts with OperatorAlreadySet and their page needs a transferOperator path instead.

CHANGELOG updated under Unreleased.