refactor: improve TypeScript patterns in test files (Group 1/8) (#8253)

## Summary
Improves type safety in test files by replacing unsafe type patterns
with proper TypeScript idioms.

## Changes
- Define typed `TestWindow` interface extending `Window` for Playwright
tests with custom properties
- Use `Partial<HTMLElement>` with single type assertion for DOM element
mocks
- Remove redundant type imports
- Fix `console.log` → `console.warn` in test fixture

## Files Changed
16 test files across browser_tests, packages, and src/components

## Test Plan
- ✅ `pnpm typecheck` passes
- ✅ No new `any` types introduced
- ✅ All pre-commit hooks pass

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8253-refactor-improve-TypeScript-patterns-in-test-files-Group-1-8-2f16d73d365081548f9ece7bcf0525ee)
by [Unito](https://www.unito.io)

---------

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

packages/shared-frontend-utils/src/formatUtil.test.ts

@@ -120,8 +120,8 @@ describe('formatUtil', () => {
       })
 
       it('should handle null and undefined gracefully', () => {
-        expect(getMediaTypeFromFilename(null as any)).toBe('image')
-        expect(getMediaTypeFromFilename(undefined as any)).toBe('image')
+        expect(getMediaTypeFromFilename(null)).toBe('image')
+        expect(getMediaTypeFromFilename(undefined)).toBe('image')
       })
 
       it('should handle special characters in filenames', () => {

packages/shared-frontend-utils/src/formatUtil.ts

@@ -537,7 +537,9 @@ export function truncateFilename(
  * @param filename The filename to analyze
  * @returns The media type: 'image', 'video', 'audio', or '3D'
  */
-export function getMediaTypeFromFilename(filename: string): MediaType {
+export function getMediaTypeFromFilename(
+  filename: string | null | undefined
+): MediaType {
   if (!filename) return 'image'
   const ext = filename.split('.').pop()?.toLowerCase()
   if (!ext) return 'image'