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/res/xml/file_paths.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- FileProvider scope for the party-screen "Share this app" APK handoff. -->
+<paths>
+    <cache-path name="share" path="share/" />
+</paths>