feat(core,app,android,ios): finish adding a first share where it pays off

- A link with a password of its own, added without one, now fails as
  "This link has a password of its own" instead of the handshake's
  refusal. The link's public flags are only asked for after a refusal.
  The desktop form turns its password toggle on.
- After the first share is added, all three clients go to the library.
  While that share is being listed, the empty library says so instead of
  offering to add a share.
- The desktop library offers posters and synopses once there is a
  library to put them on. The offer says what is sent, links to
  Settings, never turns anything on, and "Not now" is remembered.

ios/ProtonStream/UI/LibraryView.swift

@@ -145,6 +145,10 @@ struct LibraryView: View {
             LibrarySkeleton()
         } else if model.titles.isEmpty && !model.query.trimmingCharacters(in: .whitespaces).isEmpty {
             EmptyState("Nothing matches", "No title in the library is called “\(model.query.trimmingCharacters(in: .whitespaces))”.")
+        } else if model.titles.isEmpty, let activity = model.activity {
+            // A share just added: say it is being looked through, rather than
+            // offer to add the one that is already filling the page.
+            EmptyState("Looking through the share", "\(activity) Its titles appear here once it has been listed.")
         } else if model.titles.isEmpty {
             ScrollView {
                 EmptyState(

ios/ProtonStream/UI/SharesView.swift

@@ -6,6 +6,7 @@ struct SharesView: View {
     @Environment(\.scheme) private var scheme
     /// A link opened in this app: the Add form opens with it filled in.
     @Binding var incomingLink: String?
+    var onFirstShareAdded: () -> Void = {}
 
     @State private var showAdd = false
     @State private var signingIn = false
@@ -57,7 +58,10 @@ struct SharesView: View {
         }
         .sheet(isPresented: Binding(get: { showAdd || incomingLink != nil }, set: { if !$0 { showAdd = false; incomingLink = nil } })) {
             AddShareSheet(initialUrl: incomingLink ?? "") { name, url, password in
-                await model.addShare(name: name, url: url, password: password)
+                let first = model.titles.isEmpty
+                let refusal = await model.addShare(name: name, url: url, password: password)
+                if refusal == nil && first { onFirstShareAdded() }
+                return refusal
             }
         }
         .sheet(isPresented: Binding(