Web: only the page's first file replaces the start deck; close Save As after a same-name save

A file the page sent without a name opens untitled and unchanged, like the
empty start presentation, so the page's next file closed it. Host mode now
records the start presentation (start_for_host) and only the first file
that opens takes its place, as packaging/web/README.md says.

Save As > Save under the presentation's own name changed neither the path
nor the dirty flag, so the Save As page stayed open and a second click sent
the file again. The page now also compares the revision, which every save
bumps.

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

apps/deckcraft-web/src/web.rs

@@ -58,7 +58,11 @@ pub fn start() {
                     }
                     let mut app = SlideApp::new(Session::new(), services);
                     // In host mode the parent sends the presentation to edit; no sample meanwhile.
-                    if app_bridge.is_none() && !query_has("blank") {
+                    if app_bridge.is_some() {
+                        if let Err(e) = app.start_for_host() {
+                            log::error!("new presentation: {e}");
+                        }
+                    } else if !query_has("blank") {
                         if let Err(e) = deckcraft_engine::sample::open_sample(&mut app.session) {
                             log::error!("sample deck: {e}");
                         }