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.

crates/pstr-core/src/error.rs

@@ -46,6 +46,11 @@ pub enum Error {
     /// mailbox password.
     #[error("{0}")]
     SignIn(String),
+
+    /// A public link with a password of its own was opened without one. The
+    /// handshake refusal says nothing a viewer can act on; this does.
+    #[error("This link has a password of its own. Enter it to add the link.")]
+    PasswordNeeded,
 }
 
 impl Error {