Replace SelectPlus with custom component (migrate away from PrimeVue)

## Context

SelectPlus was introduced in PR #7804 as a temporary fix to override PrimeVue's Select component behavior. As part of the ongoing migration away from PrimeVue components toward VueUse, shadcn/vue, or Reka UI, this component should be replaced with a custom implementation.

## Current Implementation

- Location: `src/components/primevueOverride/SelectPlus.vue`
- Extends PrimeVue's Select component
- Overrides `onOverlayLeave` method to fix autofocus error on close

## Task

Replace SelectPlus with a custom select component that doesn't depend on PrimeVue.

## References

- PR where SelectPlus was introduced: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7804
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/7804#discussion_r2652376749
- Requested by: @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-8034-Replace-SelectPlus-with-custom-component-migrate-away-from-PrimeVue-2e76d73d365081f68e5afe251f215d57) by [Unito](https://www.unito.io)