Fetch latest subject before opening editor

[JeroenDeDauw]

The Infobox opened the SubjectEditorDialog with whatever subject data
was in the store cache. If the subject was modified in another tab,
the editor would show stale data. Now fetch the latest subject from
the server before opening the editor, matching the pattern already
used by SchemaDisplay and LayoutDisplay.

Test plan

• Open a subject page with an infobox in two tabs
• Edit the subject in the second tab and save
• In the first tab, click the edit button on the infobox
• Verify the editor shows the updated data from the second tab

Prompted by @JeroenDeDauw after noticing the inconsistency with Schema/Layout editors.
Context: the NeoWiki codebase and ADR 016 (Frontend State Management).
_{Written by Claude Code, Opus 4.6}

🤖 Generated with Claude Code

[JeroenDeDauw]

tested locally. LGTM