Refactor previewParam and rand to use URLSearchParams in imagePreviewStore

## Description

Currently in `src/stores/imagePreviewStore.ts`, the `getNodeImageUrls` function constructs preview URLs by mixing URLSearchParams with string concatenation:

```typescript
const params = new URLSearchParams(image)
if (isImage) appendCloudResParam(params, firstFilename)
return api.apiURL(`/view?${params}${previewParam}${rand}`)
```

The `previewParam` and `rand` parameters are appended as strings, while other parameters use the URLSearchParams API. This creates inconsistency in URL construction.

## Desired State

Refactor to add `previewParam` and `rand` values to the URLSearchParams object for consistent parameter handling:

```typescript
const params = new URLSearchParams(image)
if (isImage) appendCloudResParam(params, firstFilename)
// Add previewParam and rand to params instead of string concatenation
return api.apiURL(`/view?${params}`)
```

## Context

- Raised by @DrJKL in [PR #9298 comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/9298#discussion_r2875531769)
- Related to PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9298
- File: `src/stores/imagePreviewStore.ts`

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9346-Refactor-previewParam-and-rand-to-use-URLSearchParams-in-imagePreviewStore-3186d73d365081aab42cd9e7807005f0) by [Unito](https://www.unito.io)