Use Radar Eye logo in the app header + README

- App: top-bar brand mark is now the Radar Eye (drawable/vigil_logo.xml) instead of a generic shield.
- README: centered logo header (docs/logo.png, rendered from docs/logo.svg).
Release 0.1.7.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0187UiEtasyowhEBYs6s9iF5

README.md

@@ -1,7 +1,13 @@
+<div align="center">
+
+<img src="docs/logo.png" alt="VIGIL" width="96" height="96" />
+
 # VIGIL
 
 **What's been following you?**
 
+</div>
+
 A native Android (Kotlin) app that watches for personal item-trackers — Apple
 AirTags, Tile, Samsung Galaxy SmartTags, and Google Find My Device / DULT tags —
 that are **travelling with you over time**. It is the temporal counterpart to