Add a video export quality choice: Small file, Standard or High

A Quality dropdown under Video on Setup, --video-quality on the command
line, and "quality" in a project's video settings (written only when it is
not Standard). One table, videoQualities(), holds the ffmpeg settings per
codec; Standard is what every video was written with before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/gui_tests.cpp

@@ -553,6 +553,11 @@ void testVideo(Gtk::Window& window, const std::filesystem::path& dir) {
     stack->set_visible_child("setup");
     pump(100);
     check(preset->is_drawable() && duration->is_drawable(), "and its preset and length");
+    auto* quality = named<Gtk::DropDown>(window, "video-quality");
+    check(quality != nullptr && quality->is_drawable() &&
+              effector::videoQualities().at(quality->get_selected()).key ==
+                  effector::kDefaultVideoQuality,
+          "and its file quality, standard to begin with");
 
     preset->set_selected(1);  // Instagram story, the first preset
     pump(200);