Add lock_blocking() to TrackedMutex: blocks current thread using
block_in_place + futures::executor::block_on, safe for sync contexts.
Replace all try_lock() calls with lock_blocking() in slash commands,
UI rendering, and status reads. Lock hold times are fast enough that
blocking briefly is fine, and this eliminates the spurious 'lock
unavailable' paths that were never actually hit.
Kept rx_mutex.try_lock() in mod.rs (std::sync::Mutex for stderr rx).
