[Remove Vuetify from Studio] Channel not found error page

[vtushar06]

Summary

Removed Vuetify dependencies from the Channel not found error page by replacing ChannelDeletedError.vue (which used Vuetify's VBtn and AppError) with the existing ChannelNotFoundError.vue component that uses Kolibri Design System.

Changes made:

• Modified ChannelEditAppError.vue to use ChannelNotFoundError for both "not found" and "deleted" error states
• Removed ChannelDeletedError.vue file
• Removed import and component registration for ChannelDeletedError

Manual verification:

• Tested navigation to deleted channel URL shows correct error page
• Confirmed button interaction works as before

Screenshots:

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References

Closes #5473
Parent issue: #5060

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Reviewer guidance

1. Login as [email protected] with password a
2. Go to Channels > Published Channel
3. Copy the URL
4. Click "..." dropdown menu in upper right corner
5. Click "Delete channel"
6. Navigate to the copied URL
7. Verify the "Channel not found" error page displays correctly
8. Verify the "Back to home" button works and redirects to channels page

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[MisRob]

Thanks @vtushar06, we will assign a reviewer next week.