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/AppLogger.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * Structured application logger for form submissions.
  *
@@ -43,6 +44,32 @@ class AppLogger
         ], $extras));
     }
 
+    /**
+     * Log a duplicate-submission attempt.
+     *
+     * @param string $source             'admin' or 'partage'
+     * @param string $authorName         Author name from the incoming form
+     * @param int    $existingThesisId   ID of the matched existing thesis
+     * @param string $existingIdentifier Identifier of the matched thesis (e.g. "2025-003")
+     * @param array  $extras             Additional context (e.g. share_slug)
+     */
+    public function logDuplicate(
+        string $source,
+        string $authorName,
+        int    $existingThesisId,
+        string $existingIdentifier,
+        array  $extras = []
+    ): void {
+        $this->write(array_merge([
+            'source'              => $source,
+            'action'              => 'submit',
+            'status'              => 'duplicate',
+            'author'              => $authorName,
+            'existing_thesis_id'  => $existingThesisId,
+            'existing_identifier' => $existingIdentifier,
+        ], $extras));
+    }
+
     /**
      * Log a failed thesis submission.
      *