The wasm client never runs the engine's session-warning flow, so linking
the full sessionwatch package (timers, event composition) only bloats the
binary. Put the watcher behind a sessionDeadlineWatcher interface and split
the constructor by build tag: !js wires the real sessionwatch.Watcher, js
gets a no-op stub that still mirrors the deadline into the status recorder
(so the Status snapshot stays correct) but drops the timers. Removes the
sessionwatch package from the wasm dependency graph.
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