lint: add tsconfig for browser_tests, fix existing violations (#5633)

## Summary

See https://typescript-eslint.io/blog/project-service/ for context.
Creates a browser_tests specific tsconfig so that they can be linted.

Does not add a package.json script to do the linting yet, but `pnpm exec
eslint browser_tests` should work for now.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5633-lint-add-tsconfig-for-browser_tests-fix-existing-violations-2726d73d3650819d8ef2c4b0abc31e14)
by [Unito](https://www.unito.io)

tests-ui/tests/renderer/thumbnail/composables/useWorkflowThumbnail.spec.ts

@@ -1,10 +1,8 @@
 import { createPinia, setActivePinia } from 'pinia'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
-import {
-  ComfyWorkflow,
-  useWorkflowStore
-} from '@/platform/workflow/management/stores/workflowStore'
+import type { ComfyWorkflow } from '@/platform/workflow/management/stores/workflowStore'
+import { useWorkflowStore } from '@/platform/workflow/management/stores/workflowStore'
 
 vi.mock('@/renderer/core/thumbnail/graphThumbnailRenderer', () => ({
   createGraphThumbnail: vi.fn()