Fix crash when editing profile for non-existent user

[mowems]

Fix: Prevent crash when editing non-existent user profile

Problem

Accessing /profile/:id/edit with a non-existent username caused a server error due to @user being nil.

Solution

Added a guard clause in UsersController#edit to handle cases where the user is not found.

Result

The application now redirects safely with an error message instead of crashing.

Testing

• Visited /profile/fakeuser/edit
• Confirmed no server error occurs

[mowems]

It looks like the CI failures might be coming from deprecated GitHub Actions (for example actions/upload-artifact@v2), rather than anything introduced in this PR.

The changes here are focused on handling non-existent users in the profile edit flow, and everything is working as expected locally.

Happy to take a look at the workflow files and open a separate PR to help fix the CI if that would be useful.