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/public/assets/css/form.css

@@ -335,6 +335,7 @@ label:has(+ div > input:required)::after {
 
 /* ── Flash messages ─────────────────────────────────────────────────────── */
 .flash-error,
+.flash-warning,
 .flash-success {
     padding: var(--space-xs) var(--space-s);
     border-radius: 4px;
@@ -355,6 +356,12 @@ label:has(+ div > input:required)::after {
     color: var(--text-primary);
 }
 
+.flash-warning {
+    background: var(--warning-muted-bg, rgba(251,202,81,.12));
+    border-color: var(--warning-muted-border, rgba(251,202,81,.35));
+    color: var(--text-primary);
+}
+
 /* ── Share link badge ───────────────────────────────────────────────────── */
 .share-badge {
     display: inline-block;