Fix tabbing between fields not working with mixed_text field and add style for disabled 'Next' button.

[rute-santos]

Fix keyboard navigation through mixed_text metadata fields; style disabled wizard buttons

Problem

In the wizards (e.g. "Add event"), multi-value mixed_text fields such as
Presenter(s) or Contributor(s) broke keyboard navigation:

• Pressing Tab out of the field left it in edit mode, with the typed text
  held only in local state. The value was never committed, so required fields
  stayed invalid and the Next button stayed disabled with no visible
  reason.
• Tab also stopped on every value's remove ("x") button before reaching the
  next field.
• Sometimes focus was lost entirely after tabbing out, so the caret went
  nowhere.
• All RenderMultiField instances shared one module-level childRef, so the
  click-outside detection was attached to whichever field rendered last.

Separately, disabled Next/Create buttons looked the same as enabled
ones. NavigationButtons already sets inactive/disabled on them, but
nothing styled those states.

Changes

• RenderMultiField.tsx
  • childRef is now created per component with useRef. It is passed to
    EditMultiSelect as containerRef.
  • A new onBlur on the field container checks relatedTarget. When focus
    leaves the field, the typed value is committed and the field leaves edit
    mode. When focus moves inside the field, the value is committed and the
    field stays open. The handler runs on the next tick so it doesn't
    re-render while the browser is still moving focus. Clicks are still
    handled by useClickOutsideField.
  • submitValue is called through a ref, so callers that run outside the
    current render (the unmount cleanup) use the latest field value. Before,
    they could overwrite values added in the meantime.
  • The remove buttons have tabIndex={-1}, like the remove links in the
    legacy admin UI. Values can still be removed from the keyboard: pressing
    Backspace in the empty input removes the last value.
• wizardHooks.ts: adds a comment explaining why useClickOutsideField
  doesn't handle keyboard exit itself.
• _footer.scss: wizard footer buttons with .inactive or :disabled
  are shown at 50% opacity with a default cursor.

How to test

1. Open Add event and go to the metadata step.
2. Click into Presenter(s), type a name, and press Tab. The name should
   become a label, the field should close, and focus should land on the next
   field.
3. Tab through a field that has several values. Focus should not stop on the
   "x" buttons.
4. In an open multi-value field with an empty input, press Backspace. The
   last value should be removed.
5. Click inside an open field (input, datalist, existing labels). It should
   stay open. Click outside it. It should close and keep the typed value.
6. Leave a required field empty. Next should look greyed out, and it
   should look normal again once the field is filled.

AI Usage

🤖 Generated with Claude Code

[github-actions]

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[github-actions]

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[Arnei]

The remove buttons have tabIndex={-1}, like the remove links in the legacy admin UI. Values can still be removed from the keyboard: pressing Backspace in the empty input removes the last value.

I am against this part. Having important control elements be inaccessible to keyboard controls hurts accessibility. And the proposed alternative is hardly adequate. It seems very unintuitive and poses usability concerns (Want to delete the first element in the list? Welp, you'll have to delete all other elements first).

[Arnei]

I'd like better naming for these two handlers, to make it more clear which cases they are supposed to cover. Anyone reading this at a glance would likely be confused because they'd assume "blur" and "leave" to be the same thing.

[Arnei]

Is this an actual issue, or is Claude Code hallucinating problems into existence that aren't there? The code could be a lot simpler if this is not an actual issue.

[rute-santos]

I suspect you are right...

[Arnei]

Completely unrelated to the main issue this PR solves, should at least be its own commit