fix(gateway): include tail logs on abnormal exit

When the managed gateway process exits with an error, append the last buffered stdout/stderr lines to the exit log to make root causes visible.

Closes #2513

web/backend/api/gateway.go

@@ -762,7 +762,12 @@ func (h *Handler) startGatewayLocked(initialStatus string, existingPid int) (int
 	// Wait for exit in background and clean up
 	go func() {
 		if err := cmd.Wait(); err != nil {
-			logger.ErrorC("gateway", fmt.Sprintf("Gateway process exited: %v", err))
+			tail := gateway.logs.Tail(50)
+			if len(tail) > 0 {
+				logger.ErrorC("gateway", fmt.Sprintf("Gateway process exited: %v\nLast gateway logs:\n%s", err, strings.Join(tail, "\n")))
+			} else {
+				logger.ErrorC("gateway", fmt.Sprintf("Gateway process exited: %v", err))
+			}
 		} else {
 			logger.InfoC("gateway", "Gateway process exited normally")
 		}

web/backend/api/log.go

@@ -95,3 +95,35 @@ func (b *LogBuffer) RunID() int {
 
 	return b.runID
 }
+
+// Tail returns up to the last n buffered lines (newest last).
+// If n <= 0, it returns nil.
+func (b *LogBuffer) Tail(n int) []string {
+	if n <= 0 {
+		return nil
+	}
+
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+
+	if b.total == 0 || len(b.lines) == 0 {
+		return nil
+	}
+
+	if n > len(b.lines) {
+		n = len(b.lines)
+	}
+
+	result := make([]string, n)
+
+	if b.total <= b.cap {
+		copy(result, b.lines[len(b.lines)-n:])
+		return result
+	}
+
+	start := (b.total - n) % b.cap
+	for i := 0; i < n; i++ {
+		result[i] = b.lines[(start+i)%b.cap]
+	}
+	return result
+}