fix(ui): keep form modified after any failed submit, not just drafts

[ikhana]

What?

When a save request fails, the form is left marked as unmodified. This disables the "Save draft" button even though the user's unsaved changes are still sitting in the form.

Why?

setModified(false) runs after every submit request completes, regardless of whether it succeeded. The error branch then only restores the modified state when the submission was a draft (_status === 'draft').

So a failed publish, or any failed save on a collection without drafts enabled, leaves the form looking clean when it isn't. To an editor this reads as "my changes were saved": the save buttons are greyed out, so they navigate away and the work is lost.

The intent is already documented in that branch's existing comment, "keep the form as modified so the save button remains enabled for retry". It was just scoped to drafts only.

Steps to reproduce

1. Collection with drafts enabled and a required title
2. Save a draft, then edit content and clear the title
3. Click "Publish changes" — the request fails validation
4. "Save draft" is now disabled, with unsaved changes still in the form

How?

Moves setModified(true) out of the draft-only condition. If the request failed, the document was not persisted, so the form is still modified relative to what is stored and the save buttons should stay enabled for a retry.

Draft-specific behaviour is unchanged: setSubmitted(false) still only runs for drafts when validateDrafts is off, so failed draft saves continue not to trigger visible validation.

Fixes #17517