[UX]: Improve username configuration checks in Share Document Modal

[Gautam25Raj]

• Branch Name: ux/studio-share-username-check
• PR Title: [Feature] [Studio]: check for username before generating share link

Affected Apps / Packages

• Studio (apps/studio)

Is your feature request related to a problem?

Yes. When a user tries to create a public link for a document, the backend requires a username. If the user doesn't have one set, the API returns a 400 error and the client displays a generic toast: "Username required before sharing."
The user is forced to close the modal, navigate to their profile, and set a username. Instead, the ShareDocumentModal should check if the user is missing a username upfront, and display an inline error block with a direct action link to configure it in their Profile.

Describe the solution you'd like

• The modal should inspect the user's username (from useUserStore).
• If user.username is missing, the modal should show a warning state: "Username required: You need to set a username in your profile settings before you can share documents publicly."
• It should display a button "Go to Profile Settings" that redirects the user to the profile page, and disable the "Generate Public Link" form elements.

Describe alternatives you've considered

• Keep the form editable but throw a validation error inline on click. Checking upfront is cleaner.

Additional Context

• Subscribing to useUserStore gives us the current user object.

[aaditya-rathore]

i want to work on this