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/tests/Security/SecurityTest.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * Security Test Suite
  * Tests SQL injection protection and input sanitization
@@ -31,22 +32,22 @@ try {
         try {
             $results = $db->searchTheses(['query' => $query]);
             // Should return a (possibly empty) result set without throwing
-            echo "  ✓ Handled safely: " . substr($query, 0, 40) . "\n";
+            echo '  ✓ Handled safely: ' . substr($query, 0, 40) . "\n";
         } catch (Exception $e) {
             // A thrown exception is also acceptable (query rejected upstream)
-            echo "  ✓ Exception (safe): " . substr($query, 0, 40) . "\n";
+            echo '  ✓ Exception (safe): ' . substr($query, 0, 40) . "\n";
         }
     }
     echo "✓ PASS: SQL injection attempts handled safely\n\n";
 
     // Test 2: Invalid thesis ID
     echo "Test 2: Invalid Thesis ID\n";
-    $invalidIds = ["abc", "'; DROP TABLE theses;", "-1", "999999"];
-    
+    $invalidIds = ['abc', "'; DROP TABLE theses;", '-1', '999999'];
+
     foreach ($invalidIds as $id) {
         $result = $db->getThesisById($id);
         if ($result === null || $result === false) {
-            echo "  ✓ Rejected: " . $id . "\n";
+            echo '  ✓ Rejected: ' . $id . "\n";
         } else {
             throw new Exception("Invalid ID '$id' was not rejected");
         }
@@ -69,6 +70,6 @@ try {
     return true;
 
 } catch (Exception $e) {
-    echo "❌ FAIL: " . $e->getMessage() . "\n";
+    echo '❌ FAIL: ' . $e->getMessage() . "\n";
     return false;
 }