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/TfeController.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * TfeController
  *
@@ -74,11 +75,11 @@ class TfeController
         // Access type (1 = open, 2 = restricted, 3 = forbidden)
         $accessTypeId      = $this->db->getThesisAccessTypeId($thesisId) ?? 1;
         $isInterdit        = ($accessTypeId === 3);
-        
+
         // Check if restricted files feature is enabled and user has access
         $restrictedEnabled = $this->db->isRestrictedFilesEnabled();
         $hasRestrictedAccess = false;
-        
+
         if ($restrictedEnabled && $accessTypeId === 2) {
             // Check for cookie-based access
             $cookieToken = $_COOKIE['tfe_access_' . $thesisId] ?? null;
@@ -86,7 +87,7 @@ class TfeController
                 $hasRestrictedAccess = $this->db->hasValidCookieAccess($cookieToken, $thesisId);
             }
         }
-        
+
         // If access is restricted and user doesn't have valid access, hide files
         $shouldHideFiles = ($restrictedEnabled && $accessTypeId === 2 && !$hasRestrictedAccess);
 
@@ -209,7 +210,9 @@ class TfeController
 
         foreach ($jury as $member) {
             $name = $member['name'] ?? '';
-            if ($name === '') continue;
+            if ($name === '') {
+                continue;
+            }
 
             switch ($member['role']) {
                 case 'president':