v2.1.0: Add floating window overlay functionality

Major Features:
- Floating overlay window with system-level persistence
- Split decimal display (main speed prominent, decimal smaller/faded)
- Optimized content sizing for maximum readability
- Navigation icon trigger for overlay activation
- Multiple close options (overlay button + long-press navigation)

Technical Implementation:
- Added flutter_overlay_window dependency
- Android SYSTEM_ALERT_WINDOW permissions
- Overlay service configuration in AndroidManifest
- Enhanced flex ratios for better space utilization
- BoxFit.contain for improved content scaling

UX Improvements:
- Streamlined overlay launch (removed permission prompts)
- Cleaner visual hierarchy with split speed display
- Responsive design for both portrait and landscape
- Professional overlay styling matching app theme

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

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

pubspec.lock

@@ -134,6 +134,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.0.0"
+  flutter_overlay_window:
+    dependency: "direct main"
+    description:
+      name: flutter_overlay_window
+      sha256: "17988420249da68e421d1b44b511e6113d3de7a9e52c3fbd6d99730508af4580"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.0"
   flutter_test:
     dependency: "direct dev"
     description: flutter