Add SSH read timeout and PTY handling improvements

Fixes thread accumulation and blocking issues:

1. SSH read timeouts
   - Use ssh_channel_read_timeout() with 30s timeout
   - Handle SSH_AGAIN, SSH_ERROR return codes properly
   - 60s timeout for username input
   - Prevents threads blocking forever on dead connections

2. Improved PTY request handling
   - Exit loop after PTY + SHELL received
   - Don't loop on WINDOW_CHANGE during init
   - Clear termination conditions

3. Better UTF-8 validation
   - Check multi-byte read completeness
   - Skip incomplete sequences gracefully

Prevents "zombie threads" and eliminates need to restart
after extended operation.

include/ssh_server.h

@@ -23,6 +23,8 @@ typedef struct client {
     char command_output[2048];
     pthread_t thread;
     bool connected;
+    int ref_count;                   /* Reference count for safe cleanup */
+    pthread_mutex_t ref_lock;        /* Lock for ref_count */
 } client_t;
 
 /* Initialize SSH server */