Refactor CreatePost to use spaces with posts rooms

[KallynGowdy]

Description

Refactors the CreatePostForm component to work with spaces and their associated posts rooms, rather than directly with feed room IDs. This change introduces a new hook useSpacesWithPostsRoom that identifies spaces containing a room whose name ends with "-posts" and pairs each space with its posts room ID.

Key changes:

• Renamed useFeedRooms import to useSpacesWithPostsRoom and updated its implementation to return SpaceWithPostsRoom[] objects containing both spaceId and postsRoomId
• Updated CreatePostForm to track both the selected space and its associated posts room separately
• Changed state management from roomId to spaceId for the dropdown selector, with postsRoomId derived from the selected space
• Updated all message sending operations to use postsRoomId instead of roomId
• Updated UI labels and error messages to refer to "spaces" instead of "rooms" where appropriate
• Renamed handler and variable names for clarity (handleRoomSelect → handleSpaceSelect, feedRooms → spaces, etc.)

This refactoring maintains the same user-facing functionality while improving the semantic clarity of the code by distinguishing between the space (where the user selects from) and the posts room (where messages are actually sent).

Fixes #

Checklist:

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https://claude.ai/code/session_01Xskt9Qyd28qFo7vDvM5ohZ