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/AdminAuth.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * Minimal PHP session guard for the admin panel.
  *
@@ -161,8 +162,13 @@ class AdminAuth
         if (ini_get('session.use_cookies')) {
             $p = session_get_cookie_params();
             setcookie(
-                session_name(), '', time() - 86400,
-                $p['path'], $p['domain'], $p['secure'], $p['httponly']
+                session_name(),
+                '',
+                time() - 86400,
+                $p['path'],
+                $p['domain'],
+                $p['secure'],
+                $p['httponly']
             );
         }
         session_destroy();