[Story] Manually set a custom status

### Description

As a user
When the standard predefined statuses are not specific or accurate enough
Then I can set a custom status with my own text and emoji
So that my contacts can make the right decisions about when to expect a response from me or if to distract me.

For example, the custom text may include a specific date / time when the user will be back; or a more specific reason why the user is not available - are they on vacation or on a business trip.

Manually setting the custom status follows the same overall logic as using a predefined status, the only difference is the ability for the user to enter the text and choose the emoji.

**Designs**

The starting point(s) for setting a custom status are the same as for setting a standard predefined status. Instead of selecting one of the predefined statuses the user would choose _Custom_ to enter the desired emoji and text.

Web
- [Enter the emoji and the text](https://www.figma.com/design/IjNuVxaLRhe2MotMWJ9EtG/ER-237--User-status?node-id=68-36146\&t=fNgkwU0AcAjqKCgT-4) of the custom status.
- The default emoji is 😀.

Mobile
- Enter the emoji and the text \[[iOS](https://www.figma.com/design/IjNuVxaLRhe2MotMWJ9EtG/ER-237--User-status?node-id=52-26756\&t=fNgkwU0AcAjqKCgT-4) | [Android](https://www.figma.com/design/IjNuVxaLRhe2MotMWJ9EtG/ER-237--User-status?node-id=55-27134\&t=fNgkwU0AcAjqKCgT-4)] of the custom status.
- The default emoji is 😀.

**Conditions**:
- The emoji and text are both required to set a custom status.
- The text should be limited to 30 characters.

# Out of scope
- Nothing

# Questions
- [x] None

# Subtasks

### Web
-

### Android
-

### iOS
-

### Rust
- [x] https://github.com/matrix-org/matrix-rust-sdk/issues/6616

### Other
-

No implementation files or client entry points are named. Start by reviewing the linked Rust SDK subtask and the Web, Android, and iOS designs, then identify the corresponding client repositories and status-setting flows. Done means users can enter required emoji and text, with text limited to 30 characters and the default emoji set to 😀 across the requested clients.

Written by the indexing model from the issue text.