Replace the doc comments above the empty no-op stubs (bindTrayClick,
startTrayTheme, noopSessionWatcher.Dismiss/Close) with short in-body
comments explaining why each is empty.
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.
