Improve SpaceLobbyScreen with avatar caching, info display, and state persistence

[AndyBodnar]

This addresses most of the improvements from #663's follow-up issue #664.

What changed:

Room/space avatars now load from the avatar cache instead of always showing initials. When you scroll through the space lobby, avatars will progressively load as they're fetched.

Each entry now shows more info in the subtitle: join status for rooms you haven't joined yet (Invited, Left, Knocked, Banned), visibility (Public, Private, or Knock to join), and a truncated topic if one exists.

Expanded/collapsed state is now saved when you navigate away from a space and restored when you come back. So if you had a subspace expanded, it stays expanded.

What I skipped:

• The "Suggested" badge isn't possible yet because SpaceRoom in matrix-sdk-ui doesn't expose that field
• Parent space header info (showing topic/visibility in the header) would need more work to fetch the parent space details

Testing:

• cargo check, cargo clippy, cargo build --release all pass
• Couldn't do runtime testing since I don't have a Matrix account set up, but the code follows the same patterns used elsewhere in the codebase for avatar caching and state persistence

Closes #664

[kevinaboos]

Thanks @AndyBodnar, and welcome to the project! Wow, you're quick! I think you submitted this PR only a couple of hours after I created the issue, haha.

In general, things look pretty good, I appreciate that you followed the existing code usage/style patterns. Typically I wouldn't review PRs that haven't been tested at runtime, but I'll make an exception for your first submission.

I'm actually happy to accept this PR as-is since I'm about to completely restructure the SpaceLobbyScreen anyway. There are a few issues but nothing that is a serious blocker, just some things that will need to be fixed in the future:

1. Pulling the avatar from the cache does work, but there are two things that can be improved there.
   1. In the SpaceLobbyScreen's event handler, we need to explicitly handle Event::Signal by handling any pending avatar_cache updates (since we now use that cache) and then redraw if there were any. Otherwise, the avatars don't actually get shown when they're fetched, they don't show up until you close/re-open the SpaceLobbyScreen.
   2. Just an optimization: once we fetch the avatar from the cache, we should store it locally in the SpaceRoomInfo' room_avatar field.
2. As with the RoomScreen, we actually need to save the UI state when the SpaceLobbyScreen is destroyed, not just when showing a new one. Currently the state never actually gets saved, so there's nothing to restore.
3. Truncating a string (e.g., the topic) is an okay approach if you know that you always want it to be limited to, say, 50 characters, but in general it's best to allow the Label to truncate it via flow: Right and wrap: Ellipsis (which is currently broken in makepad but will be fixed shortly).
4. Displaying Public/Private for each subspace is actually a bit crowded and unnecessary. On second thought, I think we should probably just display that for the top-level main space, similar to what Element does.

Let me know if you want to fix these things in this PR, otherwise I can just merge it as-is and make the improvements soon after.

[AndyBodnar]

Thanks @AndyBodnar, and welcome to the project! Wow, you're quick! I think you submitted this PR only a couple of hours after I created the issue, haha.

In general, things look pretty good, I appreciate that you followed the existing code usage/style patterns. Typically I wouldn't review PRs that haven't been tested at runtime, but I'll make an exception for your first submission.

I'm actually happy to accept this PR as-is since I'm about to completely restructure the SpaceLobbyScreen anyway. There are a few issues but nothing that is a serious blocker, just some things that will need to be fixed in the future:

1. Pulling the avatar from the cache does work, but there are two things that can be improved there.

   1. In the SpaceLobbyScreen's event handler, we need to explicitly handle Event::Signal by handling any pending avatar_cache updates (since we now use that cache) and then redraw if there were any. Otherwise, the avatars don't actually get shown when they're fetched, they don't show up until you close/re-open the SpaceLobbyScreen.
   2. Just an optimization: once we fetch the avatar from the cache, we should store it locally in the SpaceRoomInfo' room_avatar field.

2. As with the RoomScreen, we actually need to save the UI state when the SpaceLobbyScreen is destroyed, not just when showing a new one. Currently the state never actually gets saved, so there's nothing to restore.

3. Truncating a string (e.g., the topic) is an okay approach if you know that you always want it to be limited to, say, 50 characters, but in general it's best to allow the Label to truncate it via flow: Right and wrap: Ellipsis (which is currently broken in makepad but will be fixed shortly).

4. Displaying Public/Private for each subspace is actually a bit crowded and unnecessary. On second thought, I think we should probably just display that for the top-level main space, similar to what Element does.

Let me know if you want to fix these things in this PR, otherwise I can just merge it as-is and make the improvements soon after.

should be good to merge now after you review new changes..