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/values/strings.xml

@@ -11,6 +11,7 @@
     <string name="welcome_title">Your Sovereign\nPersonal Server</string>
     <string name="welcome_subtitle">Bitcoin node, app platform, and private cloud — all in one box you control.</string>
     <string name="get_started">Get Started</string>
+    <string name="mesh_party">Mesh Party</string>
     <string name="use_https">Use HTTPS</string>
     <string name="port_label">Port (optional)</string>
     <string name="saved_servers">Saved Servers</string>