feat : Enable adding new AI model

[Manar-Elhabbal7]

Issue Description

I noticed there were a few TODOS comments in lib/screens/common_widgets/ai/dialog_add_ai_model.dart and within the AI model selector dialog related to adding and managing custom models.

what should it be solved ?

1. Complete the "Add Model" functionality, allowing users to add new model by adding its id and display name

2. Enable the "Update Models" button to properly refresh the list without need to restart

3. Refactore the selector dialog to use proper local state management so the UI updates instantly when new models are added.

I made actually and i will raise my PR soon

[rajesh-puripanda]

Completing the Add AI Model flow. I'll finish the TODOs in dialog_add_ai_model.dart — form validation, endpoint configuration, model selection, and persistence with the existing AI provider system.

[rajesh-puripanda]

@foss42 I've outlined my approach above — I'm ready to implement this. Please assign this issue to me. Will submit a clean PR once assigned.