feat(companion): share-the-app from Mesh Party + party entry on the intro screen — 0.5.8

- Party screen 'Share this app': shares this install's own APK through
  the system sheet (FileProvider over cache/share/), so a nearby friend
  gets the companion via Quick Share/Bluetooth with zero internet — the
  party-mode premise end to end.
- Intro screen: Mesh Party button under Get Started — party works with
  no node at all, so it belongs on the first screen a fresh install
  shows.
- Served APK: 0.5.8 (vc28), ready for the QR/OTA/ISO cut.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Android/app/src/main/AndroidManifest.xml

@@ -23,6 +23,18 @@
         android:usesCleartextTraffic="true"
         tools:targetApi="35">
 
+        <!-- Party-screen "Share this app": exposes the copied APK from
+             cache/share/ to the system share sheet, nothing else. -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
+
         <activity
             android:name=".MainActivity"
             android:exported="true"