docs: add a Screenshots section to the README landing page

Shows the three app screens (main / sources / settings) on the repo's front
page, reusing the existing docs/img assets. Complements the showcase site's
gallery.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jgJnzMr2bdNuWcDX2xRUR

README.md

@@ -17,6 +17,20 @@ Website: **[overwatch.netslum.io](https://overwatch.netslum.io)**  ·  Latest re
 
 ---
 
+## Screenshots
+
+<p align="center">
+  <img src="docs/img/overwatch-main.png" width="240" alt="Main screen — live threat-map circle" />
+  &nbsp;&nbsp;
+  <img src="docs/img/overwatch-sources.png" width="240" alt="Detection-source drill-down" />
+  &nbsp;&nbsp;
+  <img src="docs/img/overwatch-settings.png" width="240" alt="Settings" />
+</p>
+
+<p align="center"><sub>Live threat-map circle · source drill-down · settings</sub></p>
+
+---
+
 ## What it detects
 
 | Source | What it looks at | Where it comes from |