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/DuplicateThesisException.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Thrown when a submission would create a thesis that is too similar to an
+ * already-existing record.
+ *
+ * Carries enough information for the caller to build a useful warning message
+ * with a link to the existing thesis.
+ */
+class DuplicateThesisException extends RuntimeException
+{
+    public function __construct(
+        public readonly int    $existingThesisId,
+        public readonly string $existingIdentifier,
+        public readonly string $existingTitle,
+        public readonly string $existingAuthor,
+        public readonly int    $existingYear,
+        string $message = ''
+    ) {
+        if ($message === '') {
+            $message = "Un TFE similaire existe déjà ($existingIdentifier — $existingAuthor, $existingYear).";
+        }
+        parent::__construct($message);
+    }
+}