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>

desktop/src-tauri/src/update.rs

@@ -553,7 +553,7 @@ mod tests {
 
     /// The channel `update_check` would actually use.
     fn effective(db: &Db) -> Channel {
-        let conn = db.0.lock().expect("lock");
+        let conn = db.conn().expect("lock");
         store::pref(&conn, CHANNEL_PREF)
             .expect("read pref")
             .as_deref()
@@ -562,7 +562,7 @@ mod tests {
     }
 
     fn set_channel(db: &Db, channel: Channel) {
-        let conn = db.0.lock().expect("lock");
+        let conn = db.conn().expect("lock");
         store::set_pref(&conn, CHANNEL_PREF, channel.as_str()).expect("set pref");
     }
 
@@ -702,7 +702,7 @@ mod tests {
     }
 
     fn set_server(db: &Db, value: &str) {
-        let conn = db.0.lock().expect("lock");
+        let conn = db.conn().expect("lock");
         store::set_pref(&conn, SERVER_PREF, value).expect("set pref");
     }
 
@@ -780,7 +780,7 @@ mod tests {
         let db = db();
         assert_eq!(token_for(&db, "https://notes.example.com"), Ok(None));
         {
-            let conn = db.0.lock().expect("lock");
+            let conn = db.conn().expect("lock");
             state::set_link(&conn, "https://notes.example.com/", "tok").expect("link");
         }
         assert_eq!(