Custom send button composer

A same-frame composer whose send control is a custom button the extension's built-in send-button selectors deliberately do not match: no aria-label of "Send"/"Submit" and no data-testid="send-button", just an app-specific id and label. It exercises the per-rule submitSelectors escape hatch for issue #10415: a policy that names #custom-send blocks the send here, while a policy without it leaves this button unguarded (the built-in selectors miss it). 🔙 Visit Advanced fields.