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/theme_test.go

@@ -0,0 +1,83 @@
+package ui
+
+import (
+	"image/color"
+	"testing"
+
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/theme"
+)
+
+// The GoSentry theme must expose the brand colors on the semantically correct
+// ColorNames in each variant. These are the touchpoints a user actually sees —
+// the primary color drives buttons and the active tab, focus drives the accent —
+// so they are worth pinning against accidental edits to the palette maps.
+func TestGoSentryThemeBrandColors(t *testing.T) {
+	th := newGoSentryTheme()
+	cases := []struct {
+		name    string
+		color   fyne.ThemeColorName
+		variant int
+		want    color.Color
+	}{
+		{"light primary is teal", theme.ColorNamePrimary, 0, brandTeal},
+		{"light focus is amber", theme.ColorNameFocus, 0, brandAmber},
+		{"dark primary is lifted teal", theme.ColorNamePrimary, 1, brandTealLight},
+		{"dark focus is amber", theme.ColorNameFocus, 1, brandAmber},
+	}
+	for _, tc := range cases {
+		variant := theme.VariantLight
+		if tc.variant == 1 {
+			variant = theme.VariantDark
+		}
+		got := th.Color(tc.color, variant)
+		if got != tc.want {
+			t.Errorf("%s: Color(%s) = %v, want %v", tc.name, tc.color, got, tc.want)
+		}
+	}
+}
+
+// Unbranded color names must fall through to the base theme rather than render as
+// zero-value (transparent) colors, so the theme only recolors what it intends to.
+func TestGoSentryThemeDelegatesUnbrandedColors(t *testing.T) {
+	th := newGoSentryTheme()
+	base := theme.DefaultTheme()
+	// ScrollBar is not in either override map, so it must match the base theme.
+	got := th.Color(theme.ColorNameScrollBar, theme.VariantDark)
+	want := base.Color(theme.ColorNameScrollBar, theme.VariantDark)
+	if got != want {
+		t.Errorf("unbranded ColorNameScrollBar = %v, want base %v", got, want)
+	}
+}
+
+// themeFor maps the stored choice to the right theme: the GoSentry choice yields
+// the branded teal primary; every other value (including the empty legacy value)
+// yields the default theme, whose primary is not the brand teal.
+func TestThemeForChoice(t *testing.T) {
+	gosentry := themeFor(domain.ThemeGoSentry)
+	if got := gosentry.Color(theme.ColorNamePrimary, theme.VariantLight); got != brandTeal {
+		t.Errorf("themeFor(gosentry) primary = %v, want brand teal %v", got, brandTeal)
+	}
+	for _, choice := range []domain.Theme{domain.ThemeDefault, ""} {
+		def := themeFor(choice)
+		if got := def.Color(theme.ColorNamePrimary, theme.VariantLight); got == brandTeal {
+			t.Errorf("themeFor(%q) should not use the brand teal primary", choice)
+		}
+	}
+}
+
+// The dropdown label helpers must round-trip, and the empty/legacy value must map
+// to the Default label so the select never shows a blank option.
+func TestThemeLabelRoundTrip(t *testing.T) {
+	if got := themeFromLabel(themeLabel(domain.ThemeGoSentry)); got != domain.ThemeGoSentry {
+		t.Errorf("round-trip gosentry = %q", got)
+	}
+	if got := themeFromLabel(themeLabel(domain.ThemeDefault)); got != domain.ThemeDefault {
+		t.Errorf("round-trip default = %q", got)
+	}
+	if got := themeLabel(""); got != themeLabelDefault {
+		t.Errorf("empty theme label = %q, want %q", got, themeLabelDefault)
+	}
+}