Thread locking

"Close thread" and "Reopen thread" are now called "Lock thread" and "Unlock thread," with new padlock icons throughout.

Threads can no longer be locked while they contain an active poll. The group threads panel now defaults to showing unlocked threads only, with an "All" option to include locked ones, and pinned threads always appear at top regardless of lock state. The thread preview action menu also gained direct pin/unpin and lock/unlock shortcuts.