chatwoot: setCustomAttributes / setLabel pre-conversation queueing

[productdevbook]

Attributes set before the visitor sends their first message used to be dropped (no conversation yet). Recent PR queues them; we should document + test.

Evidence

• `fix(widget): Queue SDK-set conversation attributes and labels for first message chatwoot/chatwoot#13912` (merged) — "fix(widget): Queue SDK-set conversation attributes and labels for first message" fix(widget): Queue SDK-set conversation attributes and labels for first message chatwoot/chatwoot#13912
• `🔴🔴🔴 setLabel method in Chatwoot JS SDK not working 🔴🔴🔴 chatwoot/chatwoot#11771` — "setLabel method in Chatwoot JS SDK not working" 🔴🔴🔴 setLabel method in Chatwoot JS SDK not working 🔴🔴🔴 chatwoot/chatwoot#11771
• `Add custom attributes to conversation (not contact) through SDK chatwoot/chatwoot#4210` — "Add custom attributes to conversation (not contact)" Add custom attributes to conversation (not contact) through SDK chatwoot/chatwoot#4210
• `Disable setCustomAttributes chatwoot/chatwoot#3424` — "Disable setCustomAttributes" (overwrite semantics) Disable setCustomAttributes chatwoot/chatwoot#3424

Tasks

• `setAttribute({ scope: "contact" | "conversation", key, value })` — explicit scope
• Document overwrite behavior: top-level merge, leaf overwrite, null deletes
• Queue works for both contact (pre-identify) and conversation (pre-first-message) scopes
• Check Chatwoot version and fall back gracefully on versions before the queue fix

[productdevbook]

Implemented in this commit