cleanup: `max64` helper is redundant — use built-in `max()` (Go 1.21+)

[gitcoder89431]

Problem

internal/syncthing/client.go defines a manual max64 helper:

func max64(a, b int64) int64 {
    if a > b {
        return a
    }
    return b
}

Go 1.21 introduced built-in min and max functions that work on all ordered types including int64. This helper is dead weight.

Fix

Delete max64 and replace the two usages with the built-in:

return max(0, curIn-c.baselineIn), max(0, curOut-c.baselineOut), nil

Verify go.mod requires go 1.21 or higher (which it likely does since min() is already used in update.go and server.go).

[gitcoder89431]

Fixed in f9cf96a