Add customer inbox and sandbox conversation panel for UI mode

[wauputr4]

Goal

Make UI mode useful before a real WhatsApp/customer gateway is connected.

Scope

• Let the owner send sandbox customer messages through the same internal workflow as wasila sandbox customer.
• Show recent inbound/outbound messages for the selected sandbox customer.
• Show generated customer ID, ticket IDs, and owner notification IDs.
• Allow selecting an existing customer for follow-up testing.

Acceptance criteria

• A UI user can complete one local conversation without WhatsApp configured.
• Messages are persisted through existing storage.
• Ticket/customer references are visible after each response.
• The flow uses the same workflow path as CLI sandbox, not a separate mock response path.