feat: Implement real connection flow in Controller (#79)

[gonfva]

Replace the stub implementation in Controller with real functionality that:

• Validates configuration and credentials before connecting
• Provisions cloud infrastructure using AWS or Hetzner provider
• Establishes a WireGuard tunnel to the provisioned server
• Properly tears down resources on disconnect

Key changes:

• Add ControllerDeps struct for dependency injection
• Add ProviderFactory function type for creating providers
• Implement real doConnect() with config validation, provider creation, credential validation, key pair generation, and WireGuard connection
• Implement real doDisconnect() and cleanup() for proper resource teardown
• Wire up dependencies in main.go including credential storage on settings save
• Add comprehensive tests with mock implementations

Description

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing unit tests pass locally
• I have updated documentation if needed