feat: prevent duplicate TFE submissions with logging and user feedback

- Add DuplicateThesisException (typed, carries existing thesis metadata)
- Add Database::findDuplicateThesis(): matches on year + author + normalised
  title (exact, prefix, Levenshtein ≤10% of longer string)
- ThesisCreateController::submit() runs duplicate check before any DB write
  and throws DuplicateThesisException on match
- AppLogger::logDuplicate() writes status=duplicate entries to the JSON-lines
  log for audit purposes
- App::flash/consumeFlash extended to support 'warning' flash type
- admin/actions/formulaire.php: catches DuplicateThesisException, logs it,
  flashes an HTML warning toast with a clickable link to the existing thesis,
  and repopulates the form fields
- partage/index.php: same catch block; surfaces a plain-text flash-warning
  banner on the student form with identifier, title, and year of the match;
  form is repopulated via session
- toast.php: renders toast--warning variant
- admin.css: .toast--warning + link colour rules
- form.css: .flash-warning style for the partage form

app/src/Controllers/AboutController.php

@@ -1,13 +1,19 @@
 <?php
+
 require_once APP_ROOT . '/src/Database.php';
 require_once APP_ROOT . '/src/Parsedown.php';
 
-class AboutController {
+class AboutController
+{
     private string $defaultContent = "Ce site XAMXAM a été créé pour répertorier et valoriser les mémoires de l'erg – École de Recherches Graphiques de Bruxelles.\n\nL'objectif est à la fois d'offrir une vitrine aux projets des anciennes étudiantes et de mettre en lumière la diversité des disciplines et des parcours qui façonnent l'histoire de l'école à travers les âges, depuis près de 100 ans.";
 
-    public static function create(): self { return new self(); }
+    public static function create(): self
+    {
+        return new self();
+    }
 
-    public function handle(): array {
+    public function handle(): array
+    {
         try {
             $db = Database::getInstance();
             $aboutPage = $db->getPage('about');
@@ -18,9 +24,9 @@ class AboutController {
             $contacts = $db->getAproposContent('contacts');
             $credits  = $db->getAproposContent('credits');
             $contacts = is_array($contacts) && !empty($contacts) ? $contacts : null;
-            $credits  = is_array($credits)  && !empty($credits)  ? $credits  : null;
+            $credits  = is_array($credits)  && !empty($credits) ? $credits : null;
         } catch (Exception $e) {
-            error_log("Error loading about page: " . $e->getMessage());
+            error_log('Error loading about page: ' . $e->getMessage());
             $rawContent = $this->defaultContent;
             $contacts   = null;
             $credits    = null;