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/LicenceController.php

@@ -1,20 +1,24 @@
 <?php
+
 require_once APP_ROOT . '/src/Database.php';
 require_once APP_ROOT . '/src/Parsedown.php';
 
-class LicenceController {
-    public static function create(): self {
+class LicenceController
+{
+    public static function create(): self
+    {
         return new self();
     }
 
-    public function handle(): array {
+    public function handle(): array
+    {
         try {
             $db = Database::getInstance();
             $dbPage = $db->getPage('licenses');
             $content = $dbPage ? $dbPage['content'] : '';
             $pageTitle = $dbPage ? $dbPage['title'] : 'Licences';
         } catch (Exception $e) {
-            error_log("Error loading licence page: " . $e->getMessage());
+            error_log('Error loading licence page: ' . $e->getMessage());
             $content = '';
             $pageTitle = 'Licences';
         }