core: shared notes on the desktop and phone, and Share from the desktop

The core pulls with shares from a server offering them (protocol 6): a note
says how it is held (owner, edit, view) and who shared it, and a revoked note
leaves the device. The first such pull starts the feed over once, so notes
shared before this build arrive. The store refuses what a share doesn't allow
(view: everything; edit: anything but the text), push sends only the text of
someone else's note, and their notes stay out of trash, reminders and
reordering. Unlinking drops them.

The Share dialog's calls go to the linked server over the device token, as
Tauri commands and through the FFI. The desktop now offers Share and "Shared
with me"; unlinked, the dialog says sharing needs a server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/sync/engine.rs

@@ -47,9 +47,19 @@ pub async fn run_cycle(
     let attachment_sync = server
         .as_ref()
         .is_some_and(|s| s.has_feature("attachment_sync"));
+    // Notes shared with this account come only from a server offering `shares`, and
+    // an unanswered probe reads as "not offered", like `attachment_sync` above.
+    let shares = server.as_ref().is_some_and(|s| s.has_feature("shares"));
 
     let push = push::run(db, blobs, base_url, token, attachment_sync).await?;
-    let pull = pull::run(db, blobs, base_url, token).await?;
+    if shares {
+        // After the push, so nothing unsent is waiting when the full pull lands.
+        let conn = db.0.lock().map_err(|e| e.to_string())?;
+        if state::begin_shares(&conn).map_err(|e| e.to_string())? {
+            log::info!("the server shares notes now; pulling everything once to find them");
+        }
+    }
+    let pull = pull::run(db, blobs, base_url, token, shares).await?;
 
     if pull.clobbered_dirty > 0 {
         // Push ran first and reported success, so nothing should still have been