Fix file browser crash on non-regular files and connection leaks

- envd: reject non-regular files (devices, pipes, sockets) in GetFiles
  to prevent infinite reads from /dev/zero, /dev/urandom etc.
- host agent: add context cancellation check in ReadFileStream loop
  with proper Connect error codes
- frontend: abort in-flight file reads on file switch, directory
  navigation, and component teardown via AbortController
- frontend: guard against abort errors surfacing in UI, use try/finally
  for fileLoading state

internal/hostagent/server.go

@@ -516,6 +516,16 @@ func (s *Server) ReadFileStream(
 	// Stream file content in 64KB chunks.
 	buf := make([]byte, 64*1024)
 	for {
+		// Bail out early if the client disconnected or the context was cancelled.
+		select {
+		case <-ctx.Done():
+			if ctx.Err() == context.DeadlineExceeded {
+				return connect.NewError(connect.CodeDeadlineExceeded, ctx.Err())
+			}
+			return connect.NewError(connect.CodeCanceled, ctx.Err())
+		default:
+		}
+
 		n, err := resp.Body.Read(buf)
 		if n > 0 {
 			chunk := make([]byte, n)