fix: support paying RGB Lightning invoices with open asset amount

[bitwalt]

Summary

• Lightning invoices that set asset_id but leave asset_amount null (open-amount RGB invoices) were misclassified as plain BTC invoices in the Withdraw modal: the asset details panel was hidden, the amount input never appeared, and sendPayment was called without asset_id / asset_amount, so the payment could not settle.
• The Withdraw flow now keys the asset section in LightningInvoiceDetails off asset_id alone and shows "Not specified by invoice" for the missing amount; the form reveals the amount input for this case; validation uses the asset's precision-based minimum; handleConfirmedSubmit forwards asset_id plus the raw asset_amount (floored to int) to the node; ConfirmationModal renders the user-entered amount.
• Added withdrawModal.main.info.assetAmountRequired and withdrawModal.details.lightning.assetAmountNotSpecified keys across en/de/es/fr/it/ja/zh.

Repro (before this fix)

Decoded invoice on Regtest:

{
  "amt_msat": 3000000,
  "asset_id": "rgb:6q1pZv_O-nav3uB~-RyXxrqO-245hsX~-u4BDAQU-Szl8NUQ",
  "asset_amount": null,
  "payment_hash": "84c3a05e...0225",
  "network": "Regtest"
}

The Withdraw modal hid the RGB asset details and offered no input for the asset amount, so the payment could not be sent.

Test plan

• Paste an open-amount RGB Lightning invoice (asset_id set, asset_amount null) — asset section appears, amount field is shown, info banner asks for the asset amount.
• Enter an asset amount within channel capacity → confirmation modal shows the entered asset amount, payment settles via /sendpayment with asset_id + asset_amount.
• Paste a regular fixed-amount RGB Lightning invoice — behaviour unchanged (amount input stays hidden, asset amount comes from invoice).
• Paste a plain BTC Lightning invoice (with and without amount) — behaviour unchanged.
• pnpm type-check passes.

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