refactor(services): expose six more singletons via AppServices and convert callers

[datlechin]

Summary

Expose six additional singletons via AppServices and convert every raw .shared read in services-aware files:

Added to AppServices: appSettingsStorage, schemaProviderRegistry, aiKeyStorage, groupStorage, favoritesExpansionState, linkedFolderWatcher.

Converted call sites (15 total):

• AIChatViewModel — aiKeyStorage (1 site)
• FavoritesSidebarViewModel — favoritesExpansionState (3 sites)
• WelcomeViewModel — groupStorage, appSettingsStorage, linkedFolderWatcher (4 sites)
• MainContentCoordinator — schemaProviderRegistry (7 sites)

WelcomeViewModel.showOnboarding had to move from a default-value property initializer to the explicit init body so it could read services.appSettingsStorage (default initializers can't reference self).

Why this matters

This is the second tranche of the AppServices DI completion (audit findings 3.1 + arch 5.1). After PR #1151 closed the gap on AppEvents use inside the four already-converted files, this PR widens the container so every other singleton those files read can also flow through services. The remaining ~510 raw .shared reads will land in domain-scoped follow-up PRs (Storage, Database, MCP, etc.).

Test plan

• Open the welcome window; the onboarding card appears the first time and not on subsequent launches.
• AI Chat connects to a provider that uses an API key (e.g. Anthropic) and returns a response.
• Create / expand / collapse a SQL favorites folder; expansion state persists across window reopens.
• Connect and disconnect a database; MainContentCoordinator retains/releases the schema provider as before (no leak, no missing schema).
• swiftlint --strict clean on changed files.