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.

android/app/src/main/res/xml/pluriwave_file_paths.xml

@@ -3,6 +3,17 @@
     <files-path
         name="files"
         path="." />
+    <!--
+        path_provider's getApplicationDocumentsDirectory() maps to
+        context.getDir("flutter") -> <data>/app_flutter, a sibling of files/ that
+        no FileProvider tag covers directly. Without this root,
+        getUriForFile() throws for every radio recording and "open last
+        recording" fails. FileProvider canonicalizes roots, so the ../ hop
+        resolves to <data>/app_flutter.
+    -->
+    <files-path
+        name="app_flutter"
+        path="../app_flutter/" />
     <cache-path
         name="cache"
         path="." />