feat(gui): photo viewer rewrite

Auto-hiding controls, zoom and pan, a docked details panel, paging past the loaded photos with the library total in the counter, inline video playback, Save a Copy under the real name with an error toast, and Open With through the app chooser.

crates/pdfs-gui/src/app/main.rs

@@ -489,6 +489,8 @@ fn build_window(app: &adw::Application) {
             favorites: Cell::new(false),
             date_suppress: Cell::new(false),
             loading: Cell::new(false),
+            has_more: Cell::new(false),
+            page_waiters: RefCell::new(Vec::new()),
             width: Cell::new(0),
             photo_tex: RefCell::new(HashMap::new()),
             photo_tex_order: RefCell::new(VecDeque::new()),