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/sharing.rs

@@ -18,7 +18,7 @@ pub const NEEDS_SERVER: &str =
     "Sharing is between people on a server. Link this device to one in Sync to share notes.";
 
 fn link(db: &Db) -> Result<(String, String), String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     let link = state::read(&conn).map_err(|e| e.to_string())?;
     match (link.server_url, link.device_token) {
         (Some(url), Some(token)) => Ok((url, token)),
@@ -29,7 +29,7 @@ fn link(db: &Db) -> Result<(String, String), String> {
 /// Set the local note's `shared` flag from the server's list of its shares. Not a
 /// local edit, so it leaves `dirty` and `updated_at` alone.
 fn mark_shared(db: &Db, note_id: &str, shares: &[NoteShare]) -> Result<(), String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     conn.execute(
         "UPDATE notes SET shared = ?2 WHERE id = ?1 AND permission = 'owner'",
         params![note_id, !shares.is_empty()],