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>

packaging/web/README.md

@@ -113,8 +113,9 @@ DeckCraft doesn't learn the name the page finally stores a Save As under, so a p
 on a conflict should map later Saves of the old name itself.
 
 In host mode DeckCraft starts with an empty presentation instead of the sample, and the first
-presentation the page sends takes its place. The page guards leaving itself (DeckCraft adds no
-guard of its own), and `&author=Name` signs the user's comments.
+presentation the page sends takes its place unless the user has changed it; later ones open
+beside it. The page guards leaving itself (DeckCraft adds no guard of its own), and
+`&author=Name` signs the user's comments.
 
 ```html
 <iframe id="deck" src="/deckcraft/?host=parent&author=Ada" style="width: 100%; height: 720px; border: 0;"></iframe>