Add cancellation flow for support sessions

Server changes:
- Allow cancelling connected codes (not just pending)
- Reject agent connections with cancelled codes
- Periodic cancellation check during active sessions
- Send Disconnect message when code is cancelled

Agent changes:
- Detect cancellation via Disconnect message
- Show Windows MessageBox to notify user
- Exit cleanly without reconnecting for support sessions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/src/session/mod.rs

@@ -202,6 +202,10 @@ impl SessionManager {
 
             Some(message::Payload::Disconnect(disc)) => {
                 tracing::info!("Disconnect requested: {}", disc.reason);
+                // Check if this is a cancellation
+                if disc.reason.contains("cancelled") {
+                    return Err(anyhow::anyhow!("SESSION_CANCELLED: {}", disc.reason));
+                }
                 return Err(anyhow::anyhow!("Disconnect: {}", disc.reason));
             }