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/ui/run.go

@@ -60,6 +60,9 @@ func Run(startInTray bool) {
 		a.Run()
 		return
 	}
+	// Apply the persisted theme before building content so the window renders in
+	// the chosen theme from the first frame rather than flashing the default one.
+	applyTheme(a, svc.Store().Config.Theme)
 	content, recordStartup := newMainView(w, svc)
 	w.SetContent(content)
 	serveSingleInstance(instanceListener, w)