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/templates/admin/partials/toast.php

@@ -4,6 +4,12 @@
     <?= htmlspecialchars($flash['error']) ?>
 </p>
 <?php endif; ?>
+<?php if (!empty($flash['warning'])): ?>
+<p class="toast toast--warning" role="alert">
+    <span class="toast__icon" aria-hidden="true">⚠</span>
+    <?= $flash['warning'] /* pre-sanitised HTML allowed for the duplicate link */ ?>
+</p>
+<?php endif; ?>
 <?php if ($flash['success']): ?>
 <p class="toast toast--success" role="status">
     <span class="toast__icon" aria-hidden="true">✓</span>