Extracts the Meta JS SDK logic for the WhatsApp embedded-signup and
Facebook Page connect flows out of their settings components into two
reusable composables. No behavior change — the settings inbox-creation
flows work exactly as before; this just makes the SDK orchestration
reusable (an upcoming onboarding PR consumes them) and adds unit
coverage.
## What changed
- `useWhatsappEmbeddedSignup` — owns the embedded-signup popup (SDK
load, FB.login, and the auth-code / postMessage race).
`WhatsappEmbeddedSignup.vue` now consumes it.
- `useFacebookPageConnect` — owns FB.login (page scopes) + page fetch,
with SDK preload split from login so the popup opens within the click's
activation window. `Facebook.vue` now consumes it.
- Adds unit specs for both composables.
## Related PRs
- https://github.com/chatwoot/chatwoot/pull/14569
- https://github.com/chatwoot/chatwoot/pull/14568
- https://github.com/chatwoot/chatwoot/pull/14567
- https://github.com/chatwoot/chatwoot/pull/14649
- https://github.com/chatwoot/chatwoot/pull/14565 (Primary onboarding
PR)
