Allow pinning rooms to the top of the room list

[kalix127]

Content

Add the ability to pin rooms to the top of the room list. Pinning only affects sort order, not visibility: pinned rooms that match the active filter appear first, but pinned rooms that don't match the filter are still hidden. The most recently pinned room appears at the very top. A small pin icon shows next to the timestamp on pinned rooms.

Pin/unpin is available from:

• Long-press context menu on the room list (next to the Favourite toggle)
• Room Details screen (next to the Favourite toggle)

Pinned room IDs are stored locally per session using DataStore since the Matrix Rust SDK does not expose custom room tags.

Motivation and context

Closes #6466

Screenshots / GIFs

video-5.mp4

Tests

• Long-press a room, tap "Pin to top", verify it moves to the top of the list with a pin icon
• Pin multiple rooms, verify the most recently pinned one is at the very top
• Apply filters (People, Rooms, Unread), verify pinned rooms float to the top within the filtered set
• Apply a filter that excludes a pinned room (e.g. filter "Rooms" when the pinned room is a DM), verify it doesn't show
• Unpin a room via context menu, verify it goes back to its normal position
• Pin/unpin from Room Details screen, verify it works the same way
• Verify the pin toggle switch updates immediately in the context menu (no need to close/reopen)

Tested devices

• Physical
• Emulator
• OS version(s): Android 15

Checklist

• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR