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/public/admin/actions/formulaire.php

@@ -24,6 +24,7 @@ error_log('FILES array: ' . print_r($_FILES, true));
 
 require_once APP_ROOT . '/src/Controllers/ThesisCreateController.php';
 require_once APP_ROOT . '/src/AppLogger.php';
+require_once APP_ROOT . '/src/DuplicateThesisException.php';
 
 $logger     = new AppLogger();
 $authorName = $_POST['auteurice'] ?? 'unknown';
@@ -41,6 +42,24 @@ try {
     header('Location: ' . $redirect);
     exit();
 
+} catch (DuplicateThesisException $e) {
+    $logger->logDuplicate('admin', $authorName, $e->existingThesisId, $e->existingIdentifier);
+
+    error_log('ThesisCreateController duplicate: ' . $e->getMessage());
+
+    // Build a warning with a clickable link to the existing thesis.
+    $existingUrl  = htmlspecialchars('/admin/edit.php?id=' . $e->existingThesisId);
+    $existingRef  = htmlspecialchars($e->existingIdentifier . ' — ' . $e->existingTitle . ' (' . $e->existingYear . ')');
+    $warningHtml  = 'Doublon détecté : un TFE très similaire existe déjà. '
+                  . '<a href="' . $existingUrl . '" style="color:inherit;text-decoration:underline">' . $existingRef . '</a>'
+                  . ' Vérifiez avant de soumettre à nouveau.';
+
+    App::flash('warning', $warningHtml);
+    $_SESSION['form_data'] = $_POST;
+
+    header('Location: ../add.php');
+    exit();
+
 } catch (Exception $e) {
     $logger->logError('admin', $e->getMessage(), [
         'author'   => $authorName,