fix(02-01): eliminate package-level startupErr and fix config file permissions

- Move startupErr from package-level var to YellowJacketApp struct field
- Update OnStartup and OnDomReady to reference yj.startupErr
- Change config.Save() file permissions from 0o666 to 0o644
- Fix nlreturn lint in database/errors.go (pre-existing, blocking commit hook)

backend/config/config.go

@@ -149,7 +149,7 @@ func (c *Config) Save() error {
 		return fmt.Errorf("could not marshal config struct: %w", err)
 	}
 
-	err = os.WriteFile(c.filePath, confFileData, os.FileMode(int(0o666)))
+	err = os.WriteFile(c.filePath, confFileData, 0o644)
 	if err != nil {
 		return fmt.Errorf("could not write config file (%s): %w", c.filePath, err)
 	}