core, desktop: Db::conn everywhere, Change::default, notes_where, and shared row mappings

From the audit (#5179, core and desktop half).

- Every `db.0.lock().map_err(|e| e.to_string())?` (about 50 sites in core and
  the desktop) is now `db.conn()?`. The few sites that deliberately handle
  a poisoned lock differently, and the tests, keep their own spelling.
- push::Change derives Default, so its four constructors name only the
  fields they set.
- store: list_notes, reminders, titles and search share notes_where (ids
  from a query, each loaded through load_note). Labels share
  LABEL_SELECT/label_row, and saved filters share
  SAVED_FILTER_SELECT/saved_filter_row.

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

core/src/sync/engine.rs

@@ -67,7 +67,7 @@ pub async fn run_cycle(
         } else {
             state::SHARES
         };
-        let conn = db.0.lock().map_err(|e| e.to_string())?;
+        let conn = db.conn()?;
         if state::begin_shares(&conn, level).map_err(|e| e.to_string())? {
             log::info!("the server shares more of notes now; pulling everything once");
         }
@@ -91,7 +91,7 @@ pub async fn run_cycle(
     let retention = server.and_then(|s| s.trash_retention_days);
 
     let status = {
-        let conn = db.0.lock().map_err(|e| e.to_string())?;
+        let conn = db.conn()?;
         if let Some(days) = retention {
             state::set_server_retention(&conn, days as i64).map_err(|e| e.to_string())?;
         }