Amogh/fix: add BACK button to error dialog (#298)

[Amoghhosamane]

Description

When a user opens External Notes and clicks on a note that has no data (e.g. the note was deleted by the owner without first revoking access), an error popup is displayed via showErrDialog() in lib/widgets/err_dialogs.dart.

The popup had �arrierDismissible: false set, meaning the user must tap a button to dismiss it - but only a single OK button was present, and users could not intuitively find a way to return to the previous screen. The only workaround was navigating away via the app bar or nav bar.

What was changed:

• Added a BACK button as the first action in the AlertDialog inside showErrDialog()
• The existing OK button is retained alongside it
• Both buttons dismiss the dialog via Navigator.of(context).pop()
• Imported
  otepod/constants/ui.dart to use the canonical ButtonLabel.back constant ('BACK'), consistent with the rest of the app's button style

Why this is required:
All error popups should have an appropriately labelled button to let the user escape the error screen, as specified in issue #298.

Related Issues

Closes #298

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How To Test?

1. Log in and open External Notes
2. Find a note whose title shows a data error (note deleted by owner without revoking access)
3. Tap on that note to trigger the error popup
4. Verify the dialog now shows two buttons: BACK and OK
5. Tap BACK - dialog dismisses, user returns to previous view ?
6. Repeat step 3-4, tap OK - dialog also dismisses ?
7. Confirm neither button causes a crash or unexpected navigation

Checklist

• Screenshots included here/in linked issue BUG: Error pop up has no back button to exit error #298
• Changes adhere to the style and coding guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules
• The update contains no confidential information
• The update has no duplicated content
• No lint check errors are related to these changes (make prep or lutter analyze lib)
• Integration test dart test output or screenshot included in issue BUG: Error pop up has no back button to exit error #298
• I tested the PR on these devices:
  • Android
  • iOS
  • Linux
  • MacOS
  • Windows
  • Web
• I have identified reviewers
• The PR has been approved by reviewers

Finalising

• Merge dev into this branch
• Resolve any conflicts
• Add a one line summary into the CHANGELOG.md
• Push to the git repository and review
• Merge the PR into dev

[gjwgit]

Thanks for the very clear PR @Amoghhosamane.

But what is the difference between BACK and OK now? Don't they do the same thing and so we only need the OK?

[Amoghhosamane]

Thanks for reviewing @gjwgit.

You're right urrently both BACK and OK dismiss the dialog via Navigator.of(context).pop(), so behavior-wise they are identical.

My reasoning for adding BACK was based on issue, which specifically mentioned the lack of an obvious way to go back/exit the error popup. Since barrierDismissible: false is set, I interpreted this as needing an explicit escape action labelled BACK, consistent with other navigation patterns in the app (ButtonLabel.back)
t said, I agree the distinction between BACK and OK is unclear if both perform the same action. I'm happy to update the PR and either:

1. remove OK and keep only BACK, or
2. remove BACK and keep OK if that better matches the intended UX.
   Let me know which direction you'd prefer.