Fix spinner race condition

[holyfuchs]

uilive.New() and writer.Flush() both write to a global variable inside the uilive package, which causes data races when multiple spinners run concurrently. This adds a package-level sync.Mutex to serialize those two critical sections. The ticker loop itself is unaffected since it only reads from the writer after initialization.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
output/spinner.go	0.00%	4 Missing ⚠️

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