Refactor ScaleExtender + block scale-to-zero

[s4ros]

Summary

• Introduces a unified dao.Scaler interface and implementation to replace direct type assertions in ScaleExtender. Cleaner handling of different resource types.
• Refuses to open the scale dialog when the current replica count cannot be determined for a single-resource scale, so a user pressing OK without editing no longer silently scales the workload to zero.

Background

The dao.Scaler refactor was originally bundled into the multi-tab feature branch. Reviewer flagged it as unrelated; it has been extracted here for independent review. The feature/tabs PR contains two revert commits that neutralise these changes there.

Behavioural change in makeScaleForm

Before: when both replicasFromScaleSubresource and replicasFromReady failed, the dialog opened with "0" pre-filled and only a slog.Warn was logged. Confirming without editing → workload scaled to zero.

After:

• "" is used as the "unknown" sentinel instead of "0", so a legitimate scaled-to-zero workload still pre-fills correctly.
• For a single-resource scale where both read paths fail, the function returns an error; showScaleDialog surfaces it via the flash bar and the dialog never opens.
• Bulk-scale (len(fqns) > 1) keeps the documented 0 default — there is no single "current" value to pre-fill there, so the user must explicitly type the target.

Test plan

• go build ./...
• Manual smoke: scale a deployment with working RBAC — dialog opens with current count
• Manual smoke: scale with broken metadata read (e.g., resource removed mid-flight) — flash error, no dialog, no scale
• Manual smoke: scale a paused (replicas=0) deployment — dialog pre-fills 0 correctly
• Manual smoke: multi-select scale 2+ deployments — dialog opens with 0 default, user must type target

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