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/storage/store_test.go

@@ -171,6 +171,9 @@ func TestLoadOrCreateConfigCreatesDefaultsOnFirstRun(t *testing.T) {
 	if got.DefaultTimeoutSeconds != 30 {
 		t.Errorf("default DefaultTimeoutSeconds = %d, want 30", got.DefaultTimeoutSeconds)
 	}
+	if got.Theme != domain.ThemeDefault {
+		t.Errorf("default Theme = %q, want %q", got.Theme, domain.ThemeDefault)
+	}
 	// The function must have written the defaults to gosentry.json.
 	if _, err := os.Stat(paths.ConfigPath); err != nil {
 		t.Errorf("gosentry.json should have been created: %v", err)