fix: context propagation in LLM calls, SSE error handling, security hardening

- Propagate request context through MCP client (Request.Ctx field)
  — Client disconnects now cancel in-flight LLM API calls (both streaming and non-streaming)
  — Prevents wasted LLM tokens when user navigates away mid-response
- proxyBinance: use http.NewRequestWithContext for context-aware upstream calls
  — Client disconnects cancel Binance proxy requests
  — Distinguishes client cancellation from upstream failures
- Fix SSE parse bug in AgentChatPage: catch block was swallowing error events
  — throw new Error(data) for 'error' events was caught by the same try/catch
  — Now parse JSON separately, then handle events outside try/catch
- Add io.LimitReader to Hyperliquid coins.go JSON decoder (4MB limit)
- Use safe.ReadAllLimited in batch ticker handler for consistency
- Sanitize URL validation error in handler_ai_model.go (was leaking raw error)
- Cache agent tool definitions (built once, reused per message)
- Sort trade history by exit time for consistent ordering

mcp/client.go

@@ -503,6 +503,10 @@ func (client *Client) callWithRequestFull(req *Request) (*LLMResponse, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to create request: %w", err)
 	}
+	// Propagate request context so client disconnects cancel in-flight LLM calls
+	if req.Ctx != nil {
+		httpReq = httpReq.WithContext(req.Ctx)
+	}
 
 	resp, err := client.HTTPClient.Do(httpReq)
 	if err != nil {
@@ -541,6 +545,10 @@ func (client *Client) callWithRequest(req *Request) (string, error) {
 	if err != nil {
 		return "", fmt.Errorf("failed to create request: %w", err)
 	}
+	// Propagate request context so client disconnects cancel in-flight LLM calls
+	if req.Ctx != nil {
+		httpReq = httpReq.WithContext(req.Ctx)
+	}
 
 	resp, err := client.HTTPClient.Do(httpReq)
 	if err != nil {
@@ -686,8 +694,13 @@ func (client *Client) CallWithRequestStream(req *Request, onChunk func(string))
 
 	// Idle-timeout watchdog: cancel the request if no SSE line arrives for 60 seconds.
 	// This breaks the scanner out of an indefinitely blocking Read on a hung connection.
+	// Use request's Ctx as parent so client disconnects propagate cancellation.
 	const idleTimeout = 60 * time.Second
-	ctx, cancel := context.WithCancel(context.Background())
+	parentCtx := req.Ctx
+	if parentCtx == nil {
+		parentCtx = context.Background()
+	}
+	ctx, cancel := context.WithCancel(parentCtx)
 	defer cancel()
 	resetCh := make(chan struct{}, 1)
 	safe.GoNamed("mcp-stream-idle-watchdog", func() {