Always download SVG screenshots

The dashboard now always downloads SVG on right-click even when
PNG thumbnails are enabled, while keeping SVG as the default mode
in the docs and tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

webterm/server_test.go

@@ -552,6 +552,9 @@ func TestDashboardUsesPNGWhenEnabled(t *testing.T) {
 	if !strings.Contains(text, "screenshot.png") {
 		t.Fatalf("expected dashboard to request png screenshots when enabled")
 	}
+	if !strings.Contains(text, "sanitize_font_urls=1&download=1") || !strings.Contains(text, "screenshot.svg") {
+		t.Fatalf("expected contextmenu downloads to use svg screenshots")
+	}
 }
 
 func TestRootTerminalPageAndSparklineValidation(t *testing.T) {