Default range rings to enabled

- Set showRange to true by default in map-manager.js
- Check range rings checkbox by default in HTML
- Provides better initial user experience with distance reference

🤖 Generated with [Claude Code](https://claude.ai/code)

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

assets/static/index.html

@@ -93,7 +93,7 @@
                             <span>Site Positions</span>
                         </label>
                         <label>
-                            <input type="checkbox" id="show-range-rings">
+                            <input type="checkbox" id="show-range-rings" checked>
                             <span>Range Rings</span>
                         </label>
                         <label>

assets/static/js/modules/map-manager.js

@@ -9,7 +9,7 @@ export class MapManager {
         this.sourceMarkers = new Map();
         this.rangeCircles = new Map();
         this.showSources = true;
-        this.showRange = false;
+        this.showRange = true;
         this.selectedSource = null;
         this.heatmapLayer = null;