feat(pb): use an indeterminate spinner for the push checking phase

[aftersnow]

Problem

Follow-up to #474. The Copilot review on #474 (#474 (comment)) found that the checking phase still renders a zero transfer rate. #474 was merged before the fix landed on its branch, so this PR carries the fix.

The checking phase created a transfer bar with a nil reader. That bar has byte counters and an AverageSpeed decorator, but nothing advances them. A slow dst.Exists call therefore showed 0 B / <size> and 0 B/s under "Checking ...".

Change

• internal/pb: add ProgressBar.Spinner(prompt, name, size). It creates an indeterminate bar (total = 0, so mpb never triggers completion on its own) with a spinner filler, the prompt, the total size and the elapsed time. It has no byte counter and no speed decorator. Reset or Add replace it with a transfer bar once bytes flow. Complete gives a spinner a total so mpb can mark it done.
• pkg/backend/push.go: the checking phase calls pb.Spinner(...) instead of pb.Add(..., nil).
• Tests: TestSpinner_* cover creation, replacement by Reset, completion, the disabled path, and the rendered output (no counter, no rate). TestAdd_RendersTransferRate is the sanity check that a transfer bar with no bytes flowing does render 0.00 b / N b | 0.00 b/s, so the spinner assertion is not vacuous.

Verification

go test -race -count=1 ./internal/pb/ ./pkg/backend/
ok  	github.com/modelpack/modctl/internal/pb	1.878s
ok  	github.com/modelpack/modctl/pkg/backend	1.611s
golangci-lint run   # 0 issues

Manual end-to-end check against a local registry:3.1.2 behind a proxy that delays HEAD /v2/*/blobs/* by 2 s. Rendered output in a 220-column pty:

Checking blob => sha256:<digest> ⠋ 1.2 MB | 0s
Checking blob => sha256:<digest> ⠇ 1.2 MB | 1s
Pushing blob  => sha256:<digest> | 1.2 MB | done(0.1s)

No Checking frame contains b/s.