Setting to hide message previews in the room list, globally and per room

### Is your feature request related to a problem?

The room list shows the latest message of every conversation. On a phone that gets handed around or left face-up on a table, that exposes message content without anyone opening the app. iOS notification previews can be turned off system-wide, but there is no equivalent for the in-app room list.

Element X already has **Hide image & video previews** (see element-hq/element-x-ios#3887) for media. There is no counterpart for the text preview line.

### Describe the solution you'd like

A setting that hides the message preview line in the room list, leaving the room name, avatar, timestamp and unread indicator intact. Ideally three states, matching what iOS offers for notifications: **Always / Never / When unlocked**.

**Configurable per room as well as globally.** Element X already exposes per-room notification settings, so per-room preview settings would follow an established pattern: most conversations can keep their preview, while a specific chat stays blank in the list.

### Describe alternatives you've considered

- Disabling iOS notification previews only covers the lock screen; the in-app list still shows everything.
- Muting a room changes the unread badge but keeps the preview text.

### Additional context

Particularly relevant for self-hosted family and small-team deployments, where a single device is often shared or visible to others in the room.

Start by reviewing the referenced Element X iOS issue #3887 and the existing per-room notification settings pattern. Define how the global and per-room Always/Never/When unlocked states interact, then identify the Element X iOS room-list and settings entry points. Done means text previews are hidden according to the effective setting while room metadata and unread indicators remain visible.

Written by the indexing model from the issue text.