feat: make GoSentry light theme boldly branded

Give the light variant a soft teal window canvas with white inputs,
menus, dialogs, and buttons on top, plus teal-tinted separators, input
borders, and table headers. Cards and fields now lift off a branded
background instead of reading as a plain accent swap on gray. Text stays
dark (delegated to the base foreground) for high contrast on both the
teal canvas and the white surfaces.

Lock the teal canvas and white input background into the theme tests so
the light look cannot silently regress.

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

src/ui/theme_test.go

@@ -24,6 +24,8 @@ func TestGoSentryThemeBrandColors(t *testing.T) {
 	}{
 		{"light primary is teal", theme.ColorNamePrimary, 0, brandTeal},
 		{"light focus is amber", theme.ColorNameFocus, 0, brandAmber},
+		{"light canvas is a teal tint", theme.ColorNameBackground, 0, color.NRGBA{R: 0xDC, G: 0xEA, B: 0xED, A: 0xFF}},
+		{"light inputs stay white", theme.ColorNameInputBackground, 0, color.NRGBA{R: 0xFF, G: 0xFF, B: 0xFF, A: 0xFF}},
 		{"dark primary is lifted teal", theme.ColorNamePrimary, 1, brandTealLight},
 		{"dark focus is amber", theme.ColorNameFocus, 1, brandAmber},
 	}