Improved incoming call UI and in-call controls

[DombiX]

Is your feature request related to a problem? Please describe.
Currently, incoming calls are only presented as a standard notification. This makes them easy to overlook, especially if the device is in use or notifications are not immediately visible.
Additionally, once a call is answered, the in-call experience is limited and lacks basic controls expected in modern communication apps.

Describe the solution you'd like
would like a more robust and user-friendly calling experience, similar to other messaging apps, including:

• A full-screen incoming call UI with:
  Caller name clearly displayed
  Ringing screen (visual + sound)
  Accept / decline buttons

• Improved in-call controls:
  Microphone mute/unmute option
  Ability to switch between earpiece and loudspeaker (speakerphone)
  Clear and accessible call interface

Describe alternatives you've considered
Currently, the only indication of an incoming call is a notification, which can easily be missed. There are no built-in alternatives within the app to improve visibility or handling of calls.

Additional context
This feature would significantly improve usability and align Spixi with standard UX patterns found in modern messaging apps. It would reduce missed calls and provide users with expected call handling functionality.