fix(ui): keep the page elements editor context in sync with the pages list

[BatLeDev]

The VJSF options of the page elements editor were built once on setup, so their context froze the pages list as it was at that moment. When the editor mounted before the parent's pages fetch resolved, the "Page" selector of every link stayed empty ("Aucune donnée disponible") and no link could target a page — navigation button and menu elements included.

• vjsfOptions becomes a computed, so the context follows the pages prop instead of a setup-time snapshot
• the @ts-ignore on messages becomes a typed cast, since the comment no longer applies from inside the getter

Why: on a page draft, configuring a link towards a page was failing at random — the editor mounting before GET /api/pages resolved was enough to lose the whole list for the rest of the session.

Heads-up:

• the options object now changes identity on every recompute; useVjsf handles that (it compares options before rebuilding), but the page editor is sensitive to state-tree rebuilds — that is the part worth reviewing.
• no automated test: the failure depends on a race between the component mounting and the fetch. Reproduced by delaying GET /api/pages by 6 s, and verified before/after — empty selector, then the full list and a link persisted with a PATCH 200.