fix: replace mb_strlen/mb_substr in student-preview.php (mbstring unavailable)

TODO.md

@@ -2,6 +2,7 @@
 
 ## Fixes
 - [x] Replace `mb_strtolower` with `strtolower` in admin/index.php (mbstring not available in php8.4-fpm)
+- [x] Replace `mb_strlen`/`mb_substr` with `strlen`/`substr` in student-preview.php (same root cause)
 
 ## Features
 - [x] Student name popover preview in /repertoire (zero per-hover requests)

app/templates/partials/student-preview.php

@@ -10,8 +10,8 @@
 foreach ($theses as $t):
     $synopsis = $t['synopsis'] ?? '';
     // Truncate synopsis to ~160 chars
-    if (mb_strlen($synopsis) > 160) {
-        $synopsis = mb_substr($synopsis, 0, 157) . '…';
+    if (strlen($synopsis) > 160) {
+        $synopsis = substr($synopsis, 0, 157) . '…';
     }
     $meta = array_filter([
         $t['year']          ?? null,