Fix worker thread blocking startup by deferring IMAP connection

The eager connect() call in the worker thread could hang if the server
was unreachable, preventing the worker from ever processing commands.
Let refresh() handle the initial connection instead.

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

src/lib.rs

@@ -50,7 +50,7 @@ fn worker_loop(
     cmd_rx: mpsc::Receiver<WorkerCmd>,
     result_tx: mpsc::Sender<WorkerResult>,
 ) {
-    let mut session = connect::connect(&config).ok();
+    let mut session: Option<connect::ImapSession> = None;
 
     for cmd in &cmd_rx {
         match cmd {