feature: hide blocked players in chat

[lukepolo]

A player you block disappears from your chat right away, on every device, and comes back when you unblock them.

• useChatBlocks hands the player_blocks subscription (feature: block players #626) to socket.setHiddenAuthors(), which owns the diff so an unblock made while the layout was unmounted is still heard.
• On block, their lines leave every open room (badges, the "New" divider and unseen marks follow), their reactions come off visible lines, and the DM tab and pop-out close.
• Live :chat, history snapshots, :reaction and incoming DMs are filtered for blocked authors; a delete while hidden still sticks.
• On unblock every open room is rejoined, and a room on screen isn't re-badged by the history that comes back.
• Signing out isn't read as unblocking everyone.

Merge/deploy: pairs with 5stackgg/api#449; merge last, after #624 and #626. Until those land the diff also shows #626 and main's #609–#619.

Tests: the unblock missed on remount, blocked players' reactions (purge, snapshot and live), the on-screen re-badge, the pop-out, the sign-out guard and the incoming-DM reopen each fail with their fix reverted.
Not covered: lines an unblock restores above the "New" divider shift it up.