Render missing relation targets as red links with a create affordance

[JeroenDeDauw]

Problem

A missing relation target is now legitimate, wiki-native state, not an error:

• Referenced Subjects are kept as stub nodes (Keep referenced Subjects as stubs instead of deleting them #1080).
• relation-target-not-found is a non-blocking warning, not a blocking error
  (Validate relation statement targets server-side #1082).

But the display layer still treats a missing target as an error.
resources/ext.neowiki/src/components/Value/RelationDisplay.vue renders an unresolved target as a <span class="error">
with a "Subject not found" title.

Wanted

MediaWiki red-link semantics for relation targets:

• Render an unresolved target as a visibly not-yet-created link (red link), not an error.
• Clicking it starts a creation flow with the relation property's target schema pre-selected.

References

• In-flow target creation: Create and edit relation targets while editing a relation #971.
• Precedent from the previous codebase: https://github.com/ProfessionalWiki/LegacyNeoWiki/issues/735 (a missing target
  must not break rendering; show a placeholder).
• Tracked home for the missing-target-UI part of the dangling-references row of
  [EPIC] Review and flesh out Relations design #630.

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_{AI-authored — Claude Code, Opus 4.8 (max); detailed spec from @JeroenDeDauw via an orchestrating agent; not yet human-reviewed; code/doc citations and cross-referenced issues/PRs verified against origin/master and the GitHub API.}