BB-887: fix: Prevent navigation from invalid tabs in Unified Form

[atharvsp02]

Problem

Ticket: BB-887
Users can navigate away from a tab in the Unified Form even if they have left required fields empty or invalid. This allows them to reach the final Submit tab, but find the "Submit" button disabled with no clear indication of what went wrong.

Solution

Navigation is now blocked when leaving a tab with validation errors:

• The "Next" button is disabled.
• Forward tab navigation via the tab headers is blocked.
• A Bootstrap Danger Alert appears above the buttons explaining which tab needs attention (eg.Fix errors in 'Cover' to continue).
• Users can still freely navigate through completely empty tabs to allow them to browse the form structure before filling it out.

Areas of Impact

• unified-form.tsx - Tab validation and navigation blocking
• navbutton.tsx - Added error alert above navigation buttons
• type.ts - Added errorMessage prop to NavButtonsProps

Screenshot

Cover Tab(when some required fields are empty or invalid)

Content Tab(reached while previous tabs are empty)

Let me know if you have any suggestions or changes you’d like me to make.

Testing

• I have run the code and manually tested the changes

AI usage

• I have used AI in this PR (add more details below)
• I understand all the changes made in this PR

[faizanakhtar123]

Hi @atharvsp02 Thank you for opening this PR.... Please fill in the "AI usage" section in the PR template, it is there for a reason...
For better understanding - metabrainz/listenbrainz-server#3610