fix(frontend): label deleted-peer DMs consistently

[hmans]

What changed

• Added shared DM display helpers to derive conversation labels, avatar participants, and visibility checks from room member data.
• Updated the room list, room header, current-user bar, and quick switcher to use the shared DM logic.
• Treat direct messages whose only remaining listed member is the current user as deleted-user conversations instead of showing the current user name.
• Exclude those deleted-peer DMs from the quick switcher.
• Added unit coverage for the shared helpers and updated component tests for the new label behavior.
• Added an end-to-end regression test for account deletion to verify deleted-peer DMs render as Deleted User and do not appear in quick switcher results.

Why

When a DM peer account is deleted, the UI should not fall back to the current user name or surface a misleading self-DM label. This change makes deleted-peer conversations display consistently across the sidebar, room header, current-user bar, and quick switcher, and keeps them out of room search/navigation where they are no longer actionable.