zendesk: pre-validate key + surface helpful 400 diagnostics

[productdevbook]

Wrong key type (account vs widget) returns 400 from `/compose` with no hint.

Evidence

• `Error 400 https://ekr.zdassets.com/compose/ B3nnyL/react-zendesk#37`/`zendesk: JWT loginUser with callback + errorCallback (auth refresh flow) #20` Error 400 https://ekr.zdassets.com/compose/ B3nnyL/react-zendesk#37
• `Erro 400 compose/key dansmaculotte/vue-zendesk#54`/`Design pillar: consent mode / GDPR-safe deferred load #7` Erro 400 compose/key dansmaculotte/vue-zendesk#54
• `Constant loading when opening Zendesk dansmaculotte/vue-zendesk#38` "Constant loading when opening Zendesk" — `embeddable_blip` + `ticket_fields` Constant loading when opening Zendesk dansmaculotte/vue-zendesk#38

Tasks

• `load({ key })` — validate key is a UUID (Messenger) or ZAK (Classic) shape
• In dev mode, fetch the snippet once and surface non-200s with explanation
• Pre-warm `embeddable_blip` + `ticket_fields` on `idle` so first `show()` is snappy

[productdevbook]

Implemented in this commit