Persist Inbox "Show unread only" toggle across visits

## Problem

The Inbox's **Show unread only** toggle resets to off whenever the Inbox view is remounted. Users who prefer an unread-only Inbox have to enable it again after navigating away, reopening the Inbox, or restarting Buzz.

The current Desktop implementation stores this only as component state and initializes it to `false`:

https://github.com/block/buzz/blob/485d03a358b6d695aaf97879f3fbaf2f308d0755/desktop/src/features/home/ui/HomeView.tsx#L109-L110

## Steps to reproduce

1. Open the Inbox.
2. Enable **Show unread only**.
3. Navigate away from the Inbox and return, or restart Buzz and reopen the Inbox.

## Current behavior

**Show unread only** is off again.

## Expected behavior

Buzz remembers the user's last choice and restores the toggle to the same checked or unchecked state the next time the Inbox opens.

## Acceptance criteria

- Enabling **Show unread only** remains effective after navigating away from and back to the Inbox.
- The enabled state is restored after restarting Buzz.
- Disabling the toggle updates the saved preference so it remains disabled on subsequent visits.
- Users without a saved preference retain the current default of off.
- Automated coverage verifies preference restoration and the default/fallback behavior.