[Story] EX: Unread count badge on the Spaces tab

### Description

* As a user
* I want to see how unread rooms I have
* So that I can quickly catchup

This user story has a strong dependency on https://github.com/element-hq/element-internal/issues/646. This count is the same as the iOS app badge count.

### Design
[Android](https://www.figma.com/design/kcnHxunG1LDWXsJhaNuiHz/ER-145--Spaces-on-Element-X?node-id=1609-39631&t=U9ix6ze9LnusJhi6-0) / [iOS](https://www.figma.com/design/kcnHxunG1LDWXsJhaNuiHz/ER-145--Spaces-on-Element-X?node-id=544-60899&t=U9ix6ze9LnusJhi6-0)

### Acceptance criteria
- The badge count represents the number of unread rooms as defined in https://github.com/element-hq/element-internal/issues/646
- The badge count automatically updates when a room moves to the unread or to the read status
- No badge is displayed if there is no unread rooms

### Dependencies
- https://github.com/element-hq/element-internal/issues/646

### Out of scope
- Nothing

### Questions
- [ ]

Start by reading the definition of unread rooms in element-internal issue 646 and reviewing the Android and iOS Figma designs linked in this issue. Confirm how the shared Spaces tab is represented in the target client before implementing the story. Done means the count updates when rooms become read or unread and no badge appears when the count is zero.

Written by the indexing model from the issue text.