docs: add JSDoc to useWorkspaceTierLabel, formatTierName, and getTierLabel

## Summary

Add JSDoc comments to the exported composable and its returned helpers in `src/platform/workspace/composables/useWorkspaceTierLabel.ts`:

- `useWorkspaceTierLabel()` — describe the composable's purpose and what it returns
- `formatTierName(tier, isYearly)` — describe params and return value
- `getTierLabel(workspace)` — describe the `WorkspaceSubscriptionInfo` param shape and the nullable return

These are used across multiple components (`WorkspaceSwitcherPopover.vue`, `PricingTableWorkspace.vue`, etc.) and brief JSDoc would improve IDE discoverability and onboarding.

## References

- Raised in PR #9901: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9901#discussion_r2945814536
- Requested by @christian-byrne as a follow-up to keep the current PR narrowly scoped.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10205-docs-add-JSDoc-to-useWorkspaceTierLabel-formatTierName-and-getTierLabel-3266d73d365081338ffdc36e977289bb) by [Unito](https://www.unito.io)