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/AndroidManifest.xml

@@ -104,6 +104,23 @@
                 android:resource="@xml/pluriwave_file_paths" />
         </provider>
 
+        <!--
+            Publishes the app-private recordings folder as a browsable storage
+            root for the system file manager. MANAGE_DOCUMENTS restricts direct
+            access to the document framework (DocumentsUI); grantUriPermissions
+            lets it hand single-file access to whatever app the user picks.
+        -->
+        <provider
+            android:name=".RecordingsDocumentsProvider"
+            android:authorities="${applicationId}.recordings"
+            android:exported="true"
+            android:grantUriPermissions="true"
+            android:permission="android.permission.MANAGE_DOCUMENTS">
+            <intent-filter>
+                <action android:name="android.content.action.DOCUMENTS_PROVIDER" />
+            </intent-filter>
+        </provider>
+
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />