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-app/src/engine.rs

@@ -177,7 +177,11 @@ pub enum Event {
     /// The share the form asked for is stored; its crawl is starting.
     ShareAdded(String),
     /// The share the form asked for was refused, with why. Nothing was crawled.
-    ShareRejected(String),
+    ShareRejected {
+        error: String,
+        /// The link has a password of its own and none was given.
+        needs_password: bool,
+    },
     /// Shares opened; playback is possible from here.
     Connected {
         /// Shares that did not open, with why.
@@ -305,6 +309,16 @@ pub enum Event {
     BrowseFailed(String),
 }
 
+impl Event {
+    /// The add form's refusal of `error`.
+    fn rejected(error: &pstr_core::Error) -> Self {
+        Self::ShareRejected {
+            error: error.to_string(),
+            needs_password: matches!(error, pstr_core::Error::PasswordNeeded),
+        }
+    }
+}
+
 /// Where the Proton account stands.
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 pub enum AccountStatus {
@@ -671,7 +685,7 @@ impl Engine {
                 .await
             {
                 Ok((share, _)) => share,
-                Err(error) => return engine.emit(Event::ShareRejected(error.to_string())),
+                Err(error) => return engine.emit(Event::rejected(&error)),
             };
             engine.emit(Event::ShareAdded(share.name.clone()));
             engine.load_shares();
@@ -720,7 +734,7 @@ impl Engine {
             };
             let share = match engine.store.add_folder(&name, folder) {
                 Ok(share) => share,
-                Err(error) => return engine.emit(Event::ShareRejected(error.to_string())),
+                Err(error) => return engine.emit(Event::rejected(&error)),
             };
             engine.emit(Event::ShareAdded(share.name.clone()));
             engine.load_shares();