Error slot for TextInput

[devmount]

What changed?

TextInput's error display is now rendered through an error named slot instead of a plain {{ error }}. The error prop is still supported and is used as the slot's default content, so all existing usages keep working unchanged. When an error slot is provided, it overrides the prop and can render any content (e.g. a link) instead of plain text.

A second Error story example was added to TextInput.stories.ts showing both the plain string case and the slot case (a link inside the error message), plus two unit tests.

Why?

The error prop only accepted a plain string, which didn't allow for richer error messages containing interactive elements such as a <router-link /> (e.g. "Your session expired, please log in again"). Consumers needing this had to build a hacky workaround, manually re-implementing the invalid styling elsewhere.

Limitations and Notes

• I implemented a dedicated error-label CSS class now instead of reusing the help-label. This also fixed a regression since the error-label actually needs to be a flex container.
• Again: Only TextInput was changed. Other components with the same string-only help prop pattern (TextArea, SelectInput, CheckboxInput, BubbleSelect) are out of scope, but we might want to adjust them too.

Applicable Issues

Closes #196

Screenshots

[rwood-moz]

Should the optional error slot be added to the storybook under 'slots' or is just showing 'default' enough?

[devmount]

Should the optional error slot be added to the storybook under 'slots' or is just showing 'default' enough?

@rwood-moz Great catch, I missed that. But honestly: I couldn't find a way to make this work 😅 There seems to be no support for component slots in the @storybook/vue3-vite plugin. So I'm afraid we have to keep it that way, or maybe you have an idea?

cc @davinotdavid

[davinotdavid]

Thanks! LGTM through code diff + automated tests + local testing.

I think that it is fine not to have that in the Storybook for now / shouldn't be blocking for this to get in.