File Link: mark references whose target no longer resolves as "unavailable"

[Rikdekker]

Follow-up to #73 (observation 2 from @CedricWagner).

Problem

When a linked target is deleted (or otherwise no longer resolvable for the current user), its reference stays in the File Link field list — which is intentional (we do not prune on delete; see the reasoning in #73). But a dead entry currently looks identical to a live one, which is confusing.

Proposed fix (UI only, no event listening)

When a token's fileid does not resolve, render it in an explicit unavailable state instead of a normal clickable row:

• greyed out, non-clickable name (fall back to the basename of the cached path so there is still a label),
• a small "no longer available" hint/badge,
• keep the remove (×) button so the user can clear it deliberately.

Implementation notes

• The resolution already happens server-side via FileReferenceService::resolveMany(); a token is "unavailable" when its fileId is absent from the resolved map.
• FileLinkFieldInput.vue already declares a resolved prop shaped [{ fileId, name, path, exists }], but exists is currently never populated/used. Wire that through: mark a token unavailable when there is no resolved entry for its id.
• Backend (Application::resolveFilelinkNames() and the sidebar/grid resolve paths) currently returns only fileid => name for resolved ids. Expose enough for the frontend to distinguish "resolved" from "unresolved" (e.g. include the resolved-id set, or pass through exists).
• Pure frontend + a small payload tweak — no CacheEntryRemovedEvent/move listeners.

Acceptance

• A File Link row pointing at a deleted/inaccessible file shows as unavailable (greyed, "no longer available") and is not clickable.
• The user can still remove it.
• A live reference is unchanged.