Make the download card visible, and the events arrive at all

Two separate faults, both hiding the same feature.

The events never reached the shell. They were emitted from inside
WebKit's download delegate, which runs on the main thread, and delivering
an event means running JavaScript in a webview - which cannot happen from
in there. They are now emitted from a spawned task, like every other
event in the app that works.

And the card was drawn over the page, where it could never be seen: an
app's webview is a native view painted above everything the shell draws,
so a card over the page is a card behind the page. It now lives in the
nav, which is the shell's own - full detail when the nav is open, just
the state icon on the rail.

Verified end to end on a Gmail attachment: requested, named, saved, and
shown as "Downloaded" with a Show button that reveals it.

src-tauri/src/webviews.rs

@@ -737,7 +737,8 @@ pub fn create(
                         .unwrap()
                         .insert(url.to_string(), (id, path.clone()));
 
-                    let _ = dl_handle.emit(
+                    crate::downloads::announce(
+                        &dl_handle,
                         "download-started",
                         crate::downloads::DownloadStarted {
                             id,
@@ -755,7 +756,8 @@ pub fn create(
                     let found = state.download_paths.lock().unwrap().remove(&url.to_string());
                     if let Some((id, path)) = found {
                         state.finished_downloads.lock().unwrap().insert(id);
-                        let _ = dl_handle.emit(
+                        crate::downloads::announce(
+                            &dl_handle,
                             "download-finished",
                             crate::downloads::DownloadFinished {
                                 id,