feat(frontend): wire Azure RI exchange compatible-offerings + execute UI (refs #596)

[cristim]

Context

PR LeanerCloud/cloud-commitments-cli#1515 (closes LeanerCloud/cloud-commitments-cli#596) adds the Azure RI exchange backend endpoints:

• POST /api/ri-exchange/azure-instances/compatible-offerings (view:purchases)
• POST /api/ri-exchange/azure-instances/exchange (execute:ri-exchange)

The React frontend needs to wire these into the existing Azure exchangeable-RIs list view (parity with the AWS EC2 RI exchange flow: pick source reservations, pick target SKU/region/term, preview the priced quote, confirm max_payment_due + currency, execute).

Scope

• Add a "find compatible offerings" step after selecting source reservation(s) from the existing Azure exchangeable-RIs list.
• Surface the preview (net payable, refunds total, purchases total, policy errors) before allowing execute.
• Require an explicit max_payment_due + currency input before the execute call, matching the backend's mandatory guardrail.
• Surface 422 guardrail rejections (cap exceeded, policy errors, currency mismatch, missing net payable) as user-facing messages, not generic errors.

Notes

• Out of scope for feat(azure/ri-exchange): find compatible offerings and execute exchange (refs #473) cloud-commitments-cli#596 per that issue's own text ("a follow-up frontend PR is needed to wire the React UI").
• Backend response shapes: see AzureCompatibleOfferingsResponse / AzureExecuteExchangeResponse in internal/api/handler_ri_exchange.go.