fix: show scanning state instead of empty library on cold launch

The desktop GUI's startup library scan runs on a background thread; on a
cold filesystem cache a large library on encrypted/compressed storage takes
~60s to walk + stat, during which the content area rendered an empty video
list. That looked like a broken/empty library and prompted a needless Rescan
(which only appeared to help because the OS cache was warm by then).

Render a centered spinner + "Scanning library…" while the initial scan is
in flight (library_load_rx still open and library empty). The scan/drain
pipeline is unchanged; this is display-only.

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

src/app.rs

@@ -4218,6 +4218,28 @@ impl App {
                 .sum();
             chips + text_w("Sort:", egui::TextStyle::Body) + spacing + 8.0
         }
+        // Initial background scan still running and nothing to show yet. A cold
+        // scan of a large library on encrypted/compressed storage can take a
+        // minute (metadata stat of every sidecar, uncached after a reboot);
+        // without this the content area looks like an empty/broken library and
+        // tempts a pointless Rescan. Show that work is in progress instead.
+        if self.library_load_rx.is_some() && self.library.is_empty() {
+            ui.vertical_centered(|ui| {
+                ui.add_space(ui.available_height() * 0.32);
+                ui.add(egui::Spinner::new().size(34.0));
+                ui.add_space(14.0);
+                ui.heading("Scanning library…");
+                ui.add_space(4.0);
+                ui.label(
+                    egui::RichText::new(
+                        "First launch after a reboot can take a minute on a large library.",
+                    )
+                    .weak(),
+                );
+            });
+            ctx.request_repaint(); // keep frames coming so the drain swaps in promptly
+            return;
+        }
         if self.sidebar_view == SidebarView::Channels {
             self.channel_grid(ctx, ui);
             return;