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/local/schema.rs

@@ -291,6 +291,26 @@ ALTER TABLE attachments ADD COLUMN uploaded INTEGER NOT NULL DEFAULT 1;
 ALTER TABLE attachments ADD COLUMN upload_error TEXT;
 "#;
 
+// v11 (#5175): notes other people shared with this account.
+//
+// The feed now carries them, so a note says how it is held: `permission` is `owner`,
+// `edit` (its text may change here) or `view`. Every note already on a device is the
+// account's own, which is why the default is `owner`. `shared` is whether the owner
+// has shared it with anyone, and `shared_by_*` names the owner of one shared with us.
+//
+// `shares_synced` is whether this device has pulled with shares since it was linked.
+// The notes shared before this build sit below the cursor it already holds, so the
+// first pull from a server offering `shares` starts again from zero (see
+// `engine::run_cycle`). A pull applies idempotently, so starting over costs a
+// download and nothing else.
+const SCHEMA_V11: &str = r#"
+ALTER TABLE notes ADD COLUMN permission TEXT NOT NULL DEFAULT 'owner';
+ALTER TABLE notes ADD COLUMN shared INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE notes ADD COLUMN shared_by_id TEXT;
+ALTER TABLE notes ADD COLUMN shared_by_name TEXT;
+ALTER TABLE sync_state ADD COLUMN shares_synced INTEGER NOT NULL DEFAULT 0;
+"#;
+
 pub fn migrate(conn: &Connection) -> rusqlite::Result<()> {
     conn.execute_batch("PRAGMA foreign_keys = ON;")?;
     let version: i64 = conn.query_row("PRAGMA user_version", [], |r| r.get(0))?;
@@ -334,6 +354,10 @@ pub fn migrate(conn: &Connection) -> rusqlite::Result<()> {
         conn.execute_batch(SCHEMA_V10)?;
         conn.execute_batch("PRAGMA user_version = 10;")?;
     }
+    if version < 11 {
+        conn.execute_batch(SCHEMA_V11)?;
+        conn.execute_batch("PRAGMA user_version = 11;")?;
+    }
     Ok(())
 }
 
@@ -448,7 +472,7 @@ mod tests {
         let version: i64 = conn
             .query_row("PRAGMA user_version", [], |r| r.get(0))
             .expect("version");
-        assert_eq!(version, 10);
+        assert_eq!(version, 11);
     }
 
     /// Every attachment that predates v10 came down the feed, so it is already on the