fix: avoid nil response panic in GetUserAlive

[besnow]

Motivation

• Prevent a possible panic by avoiding calling r.StatusCode() before ensuring r and r.RawResponse are non-nil in GetUserAlive().
• Preserve existing behavior of returning an empty map[int]int and nil error for ordinary request failures and JSON unmarshal failures.

Description

• Only modified GetUserAlive() in api/panel/user.go to handle err first, then check r == nil || r.RawResponse == nil, and finally check r.StatusCode() >= 399.
• Kept the function signature unchanged and did not introduce context.Context or modify any other files or business logic.
• Preserved JSON unmarshal handling and the behavior of returning an empty map on errors, and ran gofmt on the file.

Testing

• Ran gofmt -w api/panel/user.go which completed successfully.
• Ran go test ./api/panel which passed successfully.

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Codex Task