[Email Alias] Slice 3: Username Generator UI & Pre-fill Flow

[parthashirolkar]

Parent

#70

What to build

Implement the "Generate Mask" button, selection dropdown, loading spinner, and pre-filling logic inside the Add/Edit credential username field.

Acceptance criteria

• Modify the Username input field in AddCredential and EditCredential components to include a dropdown action button.
• Render all active alias providers in the dropdown list, along with a default action to trigger the default provider.
• Display an in-input loading spinner while the network request is pending.
• Pre-fill the Username input field with the successfully generated EmailMask.
• Save the provider metadata (e.g. description/ID) in the credential notes or metadata, so the user knows where the alias is hosted.
• Write frontend component unit tests for the dropdown interactions and pre-fill transitions.

Blocked by

• [Email Alias] Slice 2: Alias Provider Settings UI Configuration #72