Add UI for keyword-based notification suppression (muted keywords)

### Your use case

### Your use case

#### What would you like to do?

I'd like to define keywords or patterns that suppress notifications for matching messages — essentially the inverse of the existing "keyword notifications" feature. When a message matches a muted keyword, it should not push a notification or bubble the room up in the room list.

#### Why would you like to do it?

I have notifications enabled for every message in certain low-traffic rooms. However, some messages don't warrant a notification — for example, automated status updates from bridges (WhatsApp, Telegram, etc.), birthday greetings, or other predictable high-volume, low-importance content. Currently there's no way to silence these without muting the entire room.

#### How would you like to achieve it?

A "Muted Keywords" list in notification settings, mirroring the existing "Keyword Notifications" feature. Keywords matching a message would suppress push notifications and optionally prevent the room from being re-sorted in the room list. Ideally this would sync across clients via push rules.

### Have you considered any alternatives?

_No response_

### Additional context

**The spec already supports this.** As explained in [matrix-org/matrix-spec#1074](https://github.com/matrix-org/matrix-spec/issues/1074#issuecomment-5314460071) by `@zecakeh` (spec collaborator):

> The `dont_notify` action was removed from the spec in v1.7, so the `actions` array should just be empty for no notifications. But other than that it should work currently with server implementations, it's probably just not configurable in clients.

This means **no spec changes are needed** — Matrix push rules already support creating rules with empty `actions` arrays to suppress notifications for matching events. The only missing piece is client-side UI to let users create and manage these rules.

This was previously tracked in [#689](https://github.com/element-hq/element-meta/issues/689) (now locked) and discussed in [#690](https://github.com/element-hq/element-meta/discussions/690). It was also reported as an iOS-specific request in [element-x-ios#6031](https://github.com/element-hq/element-x-ios/issues/6031).

Start by reviewing the existing "Keyword Notifications" feature and the notification settings UI, then read the linked Matrix push-rules context. Done means users can create and manage muted keyword or pattern rules that suppress matching notifications, with the requested room-list behavior and client syncing addressed.

Written by the indexing model from the issue text.