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>

internal/config/config.go

@@ -106,7 +106,7 @@ func Load(path string) (*Config, error) {
 
 func (c *Config) setDefaults() {
 	if c.Server.Host == "" {
-		c.Server.Host = "0.0.0.0"
+		c.Server.Host = "127.0.0.1"
 	}
 	if c.Server.Port == 0 {
 		c.Server.Port = 8080