Form error ids should derive from useId (hardcoded title-error/slug-error collide across screen + modal)

Follow-up from PR #155 (ARIA correctness).

The form error wiring in that PR gives every error `

` an id and points the control at it with `aria-describedby`. Several of those ids are hardcoded string literals — `title-error`, `slug-error`, and siblings in ProjectEdit, ProjectBuzzNew, TagEditModal, etc. When a screen and a modal that both use one of those ids are mounted at the same time (e.g. ProjectEdit with PostHelpWantedModal open), the ids collide and `aria-describedby` can resolve to the wrong element.

Derive the ids from `useId()` (as SearchBox and TagPicker already do) so each mounted form instance owns unique ids. Consider a tiny helper so the `${id}-error` convention stays in one place.

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