feat: allow host apps to register custom sections in team settings

[anilcancakir]

Summary

Allow host applications to inject custom form sections (cards) into the team settings view. Currently team settings only shows name, members, and invitations. Apps need a way to add project-specific settings (billing, integrations, preferences, etc.) without overriding the entire view.

Current Behavior

• Team settings view (MagicStarterTeamSettingsView) renders three fixed sections: General (name), Members, Invitations
• The only extensibility option is full view override via MagicStarter.view.register('teams.settings', ...), which forces the app to reimplement all existing sections

Proposed Behavior

Add a section registration API that lets host apps append custom SectionCard widgets to the team settings page:

MagicStarter.teamSettings.registerSection(
  key: 'billing',
  order: 10,
  builder: (context, team) => SectionCard(
    title: 'Billing',
    children: [
      // Custom billing form fields
    ],
  ),
);

• Sections are rendered after the built-in sections, ordered by order value
• Each section receives the current team context
• Sections can be removed by key: MagicStarter.teamSettings.removeSection('billing')
• Registration happens in the app's service provider boot(), before navigation renders

Acceptance Criteria

• MagicStarter.teamSettings.registerSection(key:, order:, builder:) API exists
• Custom sections render in team settings view after built-in sections
• Sections are sorted by order (lower = higher on page)
• removeSection(key) removes a registered section
• Duplicate key registration replaces the previous section (no duplicates)
• Works with existing feature gate (hasTeamFeatures())