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/update.rs

@@ -180,7 +180,7 @@ pub fn adopt_installer_channel(db: &Db, data_dir: &Path) {
 /// Apply `channel` unless we already applied this same marker value. Returns whether
 /// anything changed.
 fn adopt(db: &Db, channel: Channel) -> Result<bool, String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     let adopted = store::pref(&conn, CHANNEL_SEED_PREF).map_err(|e| e.to_string())?;
     // Already acted on this marker — whatever the pref says now is the user's own
     // choice, and re-applying would quietly undo it.
@@ -193,7 +193,7 @@ fn adopt(db: &Db, channel: Channel) -> Result<bool, String> {
 }
 
 fn read_channel(db: &State<'_, Db>) -> Result<Channel, String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     let raw = store::pref(&conn, CHANNEL_PREF).map_err(|e| e.to_string())?;
     Ok(raw
         .as_deref()
@@ -208,7 +208,7 @@ pub fn update_channel_get(db: State<'_, Db>) -> Result<Channel, String> {
 
 #[tauri::command]
 pub fn update_channel_set(channel: Channel, db: State<'_, Db>) -> Result<Channel, String> {
-    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    let conn = db.conn()?;
     store::set_pref(&conn, CHANNEL_PREF, channel.as_str()).map_err(|e| e.to_string())?;
     log::info!("update channel set to {}", channel.as_str());
     Ok(channel)