When a new account finishes onboarding we want to land them on a
dashboard with a working web widget already configured, branded, named,
and assigned to them, instead of an empty inbox list. This PR adds the
services that produce that widget. **No user-visible change yet:** the
services are dormant until the trigger and background job are wired up
in the follow-up PR.
## Context
Milestone 1 added `Account::BrandingEnrichmentJob`, which calls
context.dev during signup and stores brand data on
`account.custom_attributes['brand_info']`, plus the new onboarding form
that captures `domain`, `name`, `industry`, etc. Milestone 2 starts
using that data, and the first thing we want is a web widget
materialized automatically. Splitting the service layer from the
orchestration plumbing (Redis key, `onboarding_step` extension,
controller wiring, ActionCable) keeps this diff focused and lets the
LLM/widget logic merge independently.
## How to test
Run against an existing account that already has `brand_info` populated.
```ruby
account = Account.find(<account_id>)
user = account.administrators.first
inbox = WidgetCreationService.new(account, user).perform
inbox.channel.widget_color # color from brand_info, or '#1f93ff'
inbox.channel.welcome_title # brand_info[:title], or account.name
inbox.channel.welcome_tagline # LLM tagline (Enterprise + system key set),
# else brand_info[:slogan]/[:description]/nil
inbox.inbox_members.pluck(:user_id)
```
Toggle `InstallationConfig['CAPTAIN_OPEN_AI_API_KEY']` to flip between
LLM and brand-text tagline paths. To verify failure isolation, raise
inside `Captain::Llm::WidgetTaglineService#perform` and confirm widget
creation still succeeds with the fallback tagline.
