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.

templates/partials/form/select-field.php

@@ -20,12 +20,24 @@ $required    = $required    ?? false;
 $placeholder = array_key_exists('placeholder', get_defined_vars()) ? $placeholder : '';
 $id          = $id          ?? $name;
 $hint        = $hint        ?? null;
+$attrs       = $attrs       ?? [];
 ?>
 <div>
     <label for="<?= htmlspecialchars($id) ?>"><?= htmlspecialchars($label) ?></label>
+<?php
+$selectAttrStr = '';
+foreach ($attrs as $k => $v) {
+    if ($v === true) {
+        $selectAttrStr .= ' ' . htmlspecialchars($k);
+    } else {
+        $selectAttrStr .= ' ' . htmlspecialchars($k) . '="' . htmlspecialchars((string)$v) . '"';
+    }
+}
+?>
     <select id="<?= htmlspecialchars($id) ?>"
             name="<?= htmlspecialchars($name) ?>"
-            <?= $required ? 'required' : '' ?>>
+            <?= $required ? 'required' : '' ?>
+            <?= $selectAttrStr ?>>
         <?php if ($placeholder !== null): ?>
         <option value=""><?= htmlspecialchars($placeholder) ?></option>
         <?php endif; ?>
@@ -46,4 +58,4 @@ $hint        = $hint        ?? null;
     <?php endif; ?>
 </div>
 <?php
-unset($required, $placeholder, $id, $hint);
+unset($required, $placeholder, $id, $hint, $attrs, $selectAttrStr, $k, $v);