Global “Do Not Disturb” / Mute notifications toggle with keyboard shortcut

### Your use case

Currently, Element does not provide a fast way to temporarily mute all notifications globally (similar to a “Do Not Disturb” mode).

While room-level mute and notification settings exist, users often need a quick, reversible, global mute option — especially during meetings, presentations, or focused work.

There are existing discussions around time-based muting (e.g. element-meta / element-web issues), but this request focuses specifically on usability and speed, ideally via a keyboard shortcut. https://github.com/element-hq/element-meta/issues/1223

Why this is useful
- Frequent context switching (meetings, screen sharing, focus time)
- Users coming from Slack / Teams expect a quick DND toggle
- OS-level DND is not always sufficient or desired
- Improves accessibility and workflow efficiency

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No implementation files, tests, or entry points are identified in the issue. Start by reading the linked element-meta/element-web issue 1223 and compare the existing room-level mute and notification settings in the relevant Element clients. Done should include an agreed global toggle, a keyboard shortcut, and a reversible user-visible notification state.

Written by the indexing model from the issue text.