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>

desktop/src-tauri/src/capture.rs

@@ -60,7 +60,7 @@ pub struct CaptureShortcut {
 }
 
 fn stored(db: &Db) -> Result<String, String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     Ok(store::pref(&conn, SHORTCUT_PREF)
         .map_err(|e| e.to_string())?
         .unwrap_or_default())
@@ -187,7 +187,7 @@ pub fn capture_shortcut_set(
 ) -> Result<CaptureShortcut, String> {
     let wanted = shortcut.trim().to_string();
     register(&app, &wanted)?;
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     store::set_pref(&conn, SHORTCUT_PREF, &wanted).map_err(|e| e.to_string())?;
     log::info!(
         "quick capture shortcut {}",