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>
mixeme