Fix Room Asset Download Crash in Firefox By Switching to Blank Navigation

Overview & Context

Downloading a generated whiteboard asset (and any other room asset) currently uses window.location.assign(downloadUrl) in app/src/hooks/useDownloadRoomAsset.ts. In Firefox this triggers a top‑level navigation that unloads the conference. The signaling WebSocket is closed (close code 1001 "Going Away"), the conference crashes

Frontend & UI Requirements

User‑visible behavior must stay the same

Implementation Tasks

• Replace window.location.assign(downloadUrl) in useDownloadRoomAsset.ts with a _blank navigation
• Add a regression unit test asserting that window.location.assign is not called and that the download is opened in a _blank context

Definition of Done (DoD)

• Code passes linting and type checks.
• Manual verification successful (tested on Chrome and Firefox; conference WebSocket stays alive after downloading a whiteboard asset).
• Add unit tests
• Add release label after merge