WCAG 3.3.1: autofocus first invalid field on add/edit form validation failure

Add App::flashAutofocus(fieldName) and consumeAutofocus() to the thin App
helper so action handlers can identify which field caused a validation error
and the form page can move browser focus directly to it on reload.

Changes:
- src/App.php — flashAutofocus() stores field name in _flash_autofocus
  session key; consumeAutofocus() drains it and returns the name (or null)
- actions/formulaire.php — catch block maps exception messages to field
  names (auteurice, titre, synopsis, année, orientation, ap, finality,
  languages, tag, lien) and calls App::flashAutofocus()
- actions/edit.php — catch block maps common edit errors to field names
  and calls App::flashAutofocus()
- add.php — consumes the hint via App::consumeAutofocus() into
  $autofocusField; withAutofocus() helper merges autofocus=>true into
  $attrs for every field include; synopsis textarea gets inline autofocus
- edit.php — same pattern with inline ternary merges and textarea autofocus
- templates/partials/form/text-field.php — $attrs loop now emits bare
  attribute names (no ="...") when value === true, supporting autofocus,
  disabled, readonly etc. without special-casing
- templates/partials/form/select-field.php — same boolean-attr support
  added; $attrs variable initialised to [] when caller omits it

Closes WCAG 3.3.1 autofocus item in todo/04-accessibility.md.

src/App.php

@@ -83,6 +83,26 @@ class App
         $_SESSION["_flash_{$type}"] = $message;
     }
 
+    /**
+     * Store the name of the field that should receive autofocus after a
+     * validation failure (WCAG 3.3.1).
+     */
+    public static function flashAutofocus(string $fieldName): void
+    {
+        $_SESSION['_flash_autofocus'] = $fieldName;
+    }
+
+    /**
+     * Consume and return the autofocus field name, then clear it.
+     * Returns null when no autofocus hint is present.
+     */
+    public static function consumeAutofocus(): ?string
+    {
+        $field = $_SESSION['_flash_autofocus'] ?? null;
+        unset($_SESSION['_flash_autofocus']);
+        return $field;
+    }
+
     /**
      * Consume and return flash messages, then clear them from the session.
      *
@@ -117,6 +137,7 @@ class App
             $_SESSION['form_error']
         );
 
+        // Note: autofocus is consumed separately via consumeAutofocus().
         return ['error' => $error, 'success' => $success];
     }