move the ShareLink action to its own section at the bottom of the item menu

The button used to be cloned in next to "Copy Stream URL", or shoved at the top
of the menu when there was none, which put it in the middle of Jellyfin's own
command groups. Now there is a single injection path: append an actionsheetDivider
plus the action at the end of the sheet's scroller, so it reads as its own
section the way Jellyfin separates its groups.

It also stopped appearing on menus for things that are not media. The old code
resolved the item from the URL, so opening the "..." menu on a cast member inside
a movie page showed ShareLink and would have shared the movie behind it. The item
is now taken from the trigger that opened the menu (card, list row or the detail
page's own button, all of which we can walk up from) and its type is checked
against the server before anything is injected: movie, series, season, episode,
nothing else. The API refuses everything else too, so a hand-rolled request
cannot tag a person or a playlist either.

Clicking the action now dismisses the action sheet instead of leaving it stacked
under the dialog.

README.md

@@ -6,9 +6,11 @@ them an account, without them seeing the rest of your library.
 > *"here, watch this one film, the link dies tomorrow"*
 
 ShareLinks adds a **ShareLink** button (with a little share icon) to the context
-menu of any movie, episode, series or season in the Jellyfin web client. Click
-it, pick how long the link should live, and you get a URL you can send to
-anyone. When they open it, they land straight on that title, already signed in,
+menu of any movie, episode, series or season in the Jellyfin web client. It sits
+in its own section at the bottom of the menu, and it only shows up for those four
+kinds of item, never for a cast member, a studio, a library or anything else you
+cannot actually watch. Click it, pick how long the link should live, and you get
+a URL you can send to anyone. When they open it, they land straight on that title, already signed in,
 and they cannot wander off into the rest of your server.
 
 No account for them to create, no password for you to hand out, no permanent