partage: lock Année field to link's locked_year when present

When a student-access share link specifies a locked_year, the Année field
in the student-facing form is now pre-filled with that year and rendered
as disabled+readonly, with a hidden input ensuring the value is submitted.

- renderShareLinkForm() exposes  from ['locked_year']
- fieldset-academic.php checks for : if set, outputs a hidden
  input plus the visible field locked to that value; otherwise falls back
  to the normal editable field

app/public/partage/index.php

@@ -317,6 +317,10 @@ function renderShareLinkForm(string $slug, array $link): void
     $oldFn           = $shareOldFn;
     $withAutofocusFn = $shareWithAutofocusFn;
 
+    // Locked academic year: if the link specifies a locked year, pre-fill
+    // and lock the Année field so students cannot change it.
+    $lockedYear = $link['locked_year'] ?? null;
+
     // Synopsis extra: inject fieldset_synopsis help block
     ob_start();
     $helpContent = $helpFn('fieldset_synopsis');