cross-cutting: setLocale (runtime or reinit) with session-loss warning

[productdevbook]

Locale switching forces reinit on many providers; users lose the conversation context.

Evidence

• `[SOLVED] Crisp chat language doesn’t update in React SPA crisp-im/crisp-sdk-web#45` [SOLVED] Crisp chat language doesn’t update in React SPA crisp-im/crisp-sdk-web#45
• `Allow setting Crisp's chatbox language after locale change crisp-im/crisp-sdk-web#14` Allow setting Crisp's chatbox language after locale change crisp-im/crisp-sdk-web#14
• `Web Widget locale setup chatwoot/chatwoot#12624` / `#11754`
• Userlike supports `setLocale` natively

Tasks

• `setLocale(locale: string)`
• Runtime path where native API exists (Chatwoot, Userlike)
• Reinit path elsewhere — emit `onBeforeLocaleChange` so consumers can abort if active conversation
• Doc matrix per provider

[productdevbook]

Implemented in this commit