fix(favoritos): use the cross-version onReorder API and correct drag index math

The CI Flutter SDK predates v3.41 and only exposes ReorderableListView's
onReorder; the newer onReorderItem broke the build with three analyzer
errors. onReorder exists in both SDKs, so use it and compensate for its
pre-removal newIndex internally.

Fixing the call site surfaced a real ordering bug: _onReorder located the
target neighbour in the untrimmed global list, while ServicioFavoritos
.reordenar inserts into the list after the station is removed. Dragging a
station downwards past its neighbours therefore landed it one slot too far.
Adds a mid-list downward-drag test, the only case that separates the two
coordinate spaces.

test/l10n/arb_anti_copy_test.dart

@@ -44,8 +44,8 @@ void main() {
     for (final locale in _auditedLocales) {
       final arb = readArb(locale);
       for (final key in realKeys(arb)) {
-        if (!es.containsKey(key))
-          continue; // arb_parity_test's job, not this one's
+        // Missing keys are arb_parity_test's job, not this one's.
+        if (!es.containsKey(key)) continue;
         if (arb[key] == es[key] &&
             !identicalValueAllowlist.contains((locale, key))) {
           unlisted.add('$locale/$key = "${arb[key]}"');