MobileFix - Architecture: Remove Flutter imports from ViewModels

Problem Statement

Some ViewModels import Flutter UI libraries, increasing coupling with the UI layer.

Proposed Solution

Remove Flutter imports from ViewModels and rely on services/interfaces.

Implementation Approach

1. Remove package:flutter imports in ViewModels.
2. Inject services via locator or constructor.
3. Update tests accordingly.

Files to Modify

• [ ] lib/view_model/** (imports)
• [ ] test/view_model_tests/**

Code Examples

Before:

import 'package:flutter/material.dart';
class SomeViewModel extends ChangeNotifier { /* ... */ }

After:

// No Flutter UI imports in ViewModel files.
import 'package:talawa/services/navigation_service.dart';
class SomeViewModel extends ChangeNotifier {
  final NavigationService _nav = locator<NavigationService>();
}

Dependencies

• Blocked by: None
• Blocks: None

Implementation Phase

See finding description below.

Effort Estimate

Provide estimate in issue comments if not listed above.

Acceptance Criteria

• [ ] Implementation complete
• [ ] Tests added/updated (>=80% for changed units)
• [ ] All tests passing in CI
• [ ] Code review completed
• [ ] Documentation updated (if applicable)

PR Size Guardrail

If changes would touch more than 10 files, split into multiple issues/PRs (Part 1/2, Part 2/2) and link them.