Fixed dark and light themes, ui.theme auto/dark/light

Colors no longer come from the terminal's palette, whose color 0 many
color themes set to a grey that is not their background. A theme sets
tview's default styles and the color names used in texts, all chosen from
entries 16-255 of the 256-color table so terminals without 24-bit color
show them exactly. "auto" picks light or dark from the background color
the terminal reports. Settable in the Config view, applied at the next
start (widgets copy their colors when created). Replaces using the
terminal's own background and text colors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/config/config_test.go

@@ -110,6 +110,18 @@ func TestCoverStyleValidation(t *testing.T) {
 	}
 }
 
+func TestThemeValidation(t *testing.T) {
+	base := `{"radarr":{"url":"http://r","apikey":"k"},"sonarr":{"url":"http://s","apikey":"k"},"sabnzbd":{"url":"http://a","apikey":"k"},"ui":{"theme":`
+	for _, ok := range []string{`"auto"`, `"dark"`, `"light"`, `""`} {
+		if _, err := Parse([]byte(base + ok + `}}`)); err != nil {
+			t.Errorf("%s rejected: %v", ok, err)
+		}
+	}
+	if _, err := Parse([]byte(base + `"solarized"}}`)); err == nil {
+		t.Error("unknown theme accepted")
+	}
+}
+
 func TestSaveRoundTripAndPermissions(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "config.json")