fix(recordings): open the recordings folder from the system file manager

The recordings live in app-private storage (<data>/app_flutter/grabaciones),
which the Android sandbox forbids any other app from reading, so no
ACTION_VIEW on a file:// or FileProvider URI could ever open it. On top of
that, viewDirectory built an EMPTY candidate list for that path:
directoryDocumentUri returned null (path outside external storage) and
FileProvider.getUriForFile threw because pluriwave_file_paths.xml never
covered app_flutter. The loop never ran, so both entry points -- the radio
recorder and Settings -- always showed "could not open the folder".

Publish the folder as a browsable storage root via
RecordingsDocumentsProvider instead. The files never leave private storage;
the document framework asks us for them one document at a time, and the user
can browse, copy out, rename and delete straight from the file manager. The
root follows a user-configured path and falls back to the default recordings
directory. Its title reuses the already-translated recordingsFolderTitle, so
no new literal is introduced in any of the 13 locales.

Also fixes "open last recording", broken by the same missing FileProvider
root, and replaces Intent.createChooser with a bare startActivity in the
candidate loop: a chooser never throws when nothing can handle the intent, so
the first candidate always "succeeded" and the fallback chain never ran.

Device QA pending -- the provider is driven entirely by the platform's
document framework, so no unit test covers it. Each candidate logs its own
name under file_actions.viewDirectory for logcat triage.

test/servicios/servicio_alarmas_pre_notice_template_test.dart

@@ -73,6 +73,7 @@ void main() {
         'preNoticeChannelDescription',
         'openFolderTitle',
         'openRecordingTitle',
+        'recordingsRootTitle',
         'missedTitle',
       ];
       for (final clave in claves) {