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/Unit/RateLimitTest.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * Rate Limit Test
  * Tests rate limiting functionality
@@ -19,9 +20,9 @@ try {
     echo "Test 2: Check Method\n";
     $allowed = $rateLimit->check();
     if (is_bool($allowed)) {
-        echo "✓ PASS: check() returns boolean (allowed: " . ($allowed ? 'yes' : 'no') . ")\n\n";
+        echo '✓ PASS: check() returns boolean (allowed: ' . ($allowed ? 'yes' : 'no') . ")\n\n";
     } else {
-        throw new Exception("check() did not return boolean");
+        throw new Exception('check() did not return boolean');
     }
 
     // Test 3: Headers method
@@ -37,7 +38,7 @@ try {
     if (is_int($resetTime) && $resetTime >= 0) {
         echo "✓ PASS: getResetTime() returns valid value ($resetTime seconds)\n\n";
     } else {
-        throw new Exception("Invalid reset time");
+        throw new Exception('Invalid reset time');
     }
 
     // Test 5: Cleanup method
@@ -49,6 +50,6 @@ try {
     return true;
 
 } catch (Exception $e) {
-    echo "❌ FAIL: " . $e->getMessage() . "\n";
+    echo '❌ FAIL: ' . $e->getMessage() . "\n";
     return false;
 }