fix: Windows compatibility and localhost binding

**Windows compatibility:**
- Added filepath.Join for all template and static paths
- Fixes "path not found" errors on Windows

**Localhost binding:**
- Changed default host from 0.0.0.0 to 127.0.0.1
- Browser always opens on 127.0.0.1 (localhost)
- Setup mode now listens on 127.0.0.1:8080
- Updated config.example.yaml with comment about 0.0.0.0

This ensures the app works correctly on Windows and opens
browser on the correct localhost address.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config.example.yaml

@@ -2,7 +2,7 @@
 # Copy this file to config.yaml and update values
 
 server:
-  host: "0.0.0.0"
+  host: "127.0.0.1"  # Use 0.0.0.0 to listen on all interfaces
   port: 8080
   mode: "release"  # debug | release
   read_timeout: "30s"