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>

src/types/index.ts

@@ -64,6 +64,17 @@ export type {
   ToastMessageOptions
 }
 
+interface CapturedMessages {
+  clientFeatureFlags: { type: string; data: Record<string, unknown> } | null
+  serverFeatureFlags: Record<string, unknown> | null
+}
+
+interface AppReadiness {
+  featureFlagsReceived: boolean
+  apiInitialized: boolean
+  appInitialized: boolean
+}
+
 declare global {
   interface Window {
     /** For use by extensions and in the browser console. Where possible, import `app` from '@/scripts/app' instead. */
@@ -71,5 +82,11 @@ declare global {
 
     /** For use by extensions and in the browser console. Where possible, import `app` and access via `app.graph` instead. */
     graph?: unknown
+
+    /** For use in tests to capture WebSocket messages */
+    __capturedMessages?: CapturedMessages
+
+    /** For use in tests to track app initialization state */
+    __appReadiness?: AppReadiness
   }
 }