refactor: split settings page into section modules
Split the large settings.gleam (~1100 lines) into focused section modules:
- settings/basic.gleam - domain authority, relay URL, PLC directory, etc.
- settings/lexicons.gleam - lexicon ZIP upload
- settings/oauth_clients.gleam - OAuth client CRUD
- settings/admin.gleam - admin management
- settings/danger_zone.gleam - reset functionality
Also adds:
- settings/types.gleam - shared Model and Msg types (solves circular imports)
- settings/helpers.gleam - shared render_alert helper
Updates quickslice_client.gleam to import types from settings/types module.