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>

server/src/support_codes.rs

@@ -176,11 +176,11 @@ impl SupportCodeManager {
         }
     }
 
-    /// Cancel a code
+    /// Cancel a code (works for both pending and connected)
     pub async fn cancel_code(&self, code: &str) -> bool {
         let mut codes = self.codes.write().await;
         if let Some(support_code) = codes.get_mut(code) {
-            if support_code.status == CodeStatus::Pending {
+            if support_code.status == CodeStatus::Pending || support_code.status == CodeStatus::Connected {
                 support_code.status = CodeStatus::Cancelled;
                 return true;
             }
@@ -188,6 +188,18 @@ impl SupportCodeManager {
         false
     }
 
+    /// Check if a code is cancelled
+    pub async fn is_cancelled(&self, code: &str) -> bool {
+        let codes = self.codes.read().await;
+        codes.get(code).map(|c| c.status == CodeStatus::Cancelled).unwrap_or(false)
+    }
+
+    /// Check if a code is valid for connection (exists and is pending)
+    pub async fn is_valid_for_connection(&self, code: &str) -> bool {
+        let codes = self.codes.read().await;
+        codes.get(code).map(|c| c.status == CodeStatus::Pending).unwrap_or(false)
+    }
+
     /// List all codes (for dashboard)
     pub async fn list_codes(&self) -> Vec<SupportCode> {
         let codes = self.codes.read().await;