feat: selectable branded GoSentry color theme

Add a custom Fyne theme derived from the logo and app icon (deep teal
primary, amber accent, branded job-status colors) with light and dark
variants, and let the user choose between it and Fyne's default theme
from Settings. The dark variant uses deep-teal surfaces to echo the app
icon.

The choice is persisted as a new Config.Theme field ("default" /
"gosentry"), applied at startup before the first frame and live-previewed
when picked in Settings. Empty/legacy configs normalize to the default
theme so existing installs keep the original look.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/operations.go

@@ -396,5 +396,10 @@ func validateConfig(config domain.Config) error {
 	if config.DefaultTimeoutSeconds <= 0 {
 		return errors.New("default timeout must be a positive number of seconds")
 	}
+	// Empty Theme is accepted and normalized to the default on load, so older
+	// configs (and hand-built ones) stay valid without an explicit theme.
+	if config.Theme != "" && config.Theme != domain.ThemeDefault && config.Theme != domain.ThemeGoSentry {
+		return errors.New("theme must be 'default' or 'gosentry'")
+	}
 	return nil
 }