feat: add per-account lock file protection via fs2

Prevent running multiple tele-tui instances with the same account by
using advisory file locks (flock). Lock is acquired before raw mode so
errors print to normal terminal. Account switching acquires new lock
before releasing old. Also log set_tdlib_parameters errors via tracing
instead of silently discarding them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tdlib/client.rs

@@ -665,7 +665,7 @@ impl TdClient {
         let db_path_str = new_client.db_path.to_string_lossy().to_string();
 
         tokio::spawn(async move {
-            let _ = functions::set_tdlib_parameters(
+            if let Err(e) = functions::set_tdlib_parameters(
                 false,
                 db_path_str,
                 "".to_string(),
@@ -682,7 +682,10 @@ impl TdClient {
                 env!("CARGO_PKG_VERSION").to_string(),
                 new_client_id,
             )
-            .await;
+            .await
+            {
+                tracing::error!("set_tdlib_parameters failed on recreate: {:?}", e);
+            }
         });
 
         // 4. Replace self