Every lock of the store goes through Db::conn

Db::conn is documented as the one way to take the lock, yet five production and
test sites reached past it with db.0.lock(): the startup summary, the desktop's
trash sweep and config_get, and tests in sharing and update. All five now call
conn().

DRY pass #2, batch 2, F8 (#5372).

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

core/src/sync/sharing.rs

@@ -105,7 +105,7 @@ mod tests {
     fn the_shared_flag_follows_the_servers_answer_without_dirtying_the_note() {
         let db = db();
         {
-            let conn = db.0.lock().unwrap();
+            let conn = db.conn().unwrap();
             conn.execute(
                 "INSERT INTO notes (id, body, created_at, updated_at, dirty)
                       VALUES ('n1', 'x', '2026-01-01', '2026-01-01', 0)",
@@ -125,7 +125,7 @@ mod tests {
             created_at: None,
         };
         let read = |db: &Db| -> (bool, i64) {
-            let conn = db.0.lock().unwrap();
+            let conn = db.conn().unwrap();
             conn.query_row("SELECT shared, dirty FROM notes WHERE id = 'n1'", [], |r| {
                 Ok((r.get(0)?, r.get(1)?))
             })