feat: add crypto payment options to dashboard Stripe modal (CPL-274)

Switch the Add Funds modal from Stripe Card Element to Payment Element,
which auto-renders any payment methods enabled on the Stripe account
(card and supported crypto: USDC, USDP, ETH, SOL). Backend billing
endpoints already support crypto, so this is frontend-only.

Flow: user picks an amount → Continue creates a PaymentIntent and mounts
the Payment Element with its client_secret → Pay calls
stripe.confirmPayment with redirect: 'if_required'. Card stays inline;
crypto methods redirect to the wallet/Stripe-hosted flow and return to
the dashboard with ?payment_intent=...&redirect_status=...; the new
handleBillingReturn() picks that up on auth-ready, calls
confirm_payment, and refreshes the balance.

Carries forward the session-protection patterns the rest of billing.js
adopted in CPL-285/286: every handler captures billingAuthKey() up
front, uses ensureAbortController(), re-checks the captured key after
each await, attaches walletAuthHeader via billingRequestOptions in
sovereign mode, swallows AbortError, and evicts the wallet-auth cache
on auth errors. Also primes the wallet-auth signature when the modal
opens in sovereign mode so the user only sees one popup per session.

Payment Element is configured with
fields.billingDetails.address='never' so the address form is never
rendered; confirmPayment passes a default billing country in
payment_method_data so card AVS still works.

handleBillingReturn treats Stripe's `processing` redirect_status as a
non-error and shows "credits will appear once settled"; only `failed`,
`canceled`, and `requires_*` surface as errors. The
confirmPayment-backend-failure fallback now shows the pending-credit
message via the top-level status banner so closeBillingModal() can't
swallow it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: GTC6244

lit-static/dapps/dashboard/app.js

@@ -5,7 +5,7 @@
 
 import { isAuthenticated, setTheme, getTheme, logOut, setOnAuthReady, updateStatCards, initLogin, setUsageKeyOverride, toggleOverrideEnabled, updateUsageKeyOverrideUI, setChainSecuredRpcUrl, toggleChainSecuredRpcPanel, updateChainSecuredRpcUrlUI, getMode, getApiKey, convertToChainSecured, changeChainSecuredOwnership } from './auth.js';
 import { initModalClose, initConfirmClose, showStatus, hideStatus, logError } from './ui-utils.js';
-import { initBilling } from './billing.js';
+import { initBilling, handleBillingReturn } from './billing.js';
 import { initGroups, loadGroups } from './groups.js';
 import { initKeys, loadUsageKeys } from './keys.js';
 import { initActions, loadActions } from './actions.js';
@@ -281,6 +281,7 @@ setOnAuthReady(() => {
   refreshConvertVisibility();
   refreshChangeOwnershipVisibility();
   updateChainSecuredRpcUrlUI();
+  handleBillingReturn();
 });
 
 // ----- Init -----