refactor(Tag): rename 'shape' prop to 'variant' in tag.variants.ts for clarity

## Summary

In `src/components/chip/tag.variants.ts`, the CVA configuration uses a `shape` dimension with values `square`, `rounded`, and `overlay`. Both `square` and `overlay` share the same underlying shape (`rounded-sm`), meaning `shape` is not descriptive enough — `variant` would better capture the semantic difference between these options.

## Suggested Change

Rename the `shape` prop to `variant` in `tag.variants.ts`, `Tag.vue`, and all consumers (`WorkflowTemplateSelectorDialog.vue`, `SampleModelSelector.vue`, `Card.stories.ts`, `BaseModalLayout.stories.ts`, `Tag.stories.ts`, `Tag.test.ts`).

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10650
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10650#discussion_r3005094366
- Raised by: @AustinMroz
- Requested by: @dante01yoon

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10691-refactor-Tag-rename-shape-prop-to-variant-in-tag-variants-ts-for-clarity-3316d73d36508190bacfe29f643a3202) by [Unito](https://www.unito.io)