* fix(desktop): keep streaming painting in unfocused secondary chat windows
The chat transcript streams to screen through a requestAnimationFrame-gated
flush, which Chromium pauses for blurred/occluded windows. The primary window
opted out with `backgroundThrottling: false`, but the secondary "session
windows" (cmd-click pop-out, new-session, subagent-watch) hand-copied their
webPreferences and silently lost that flag — so a streamed answer in one of them
stalled until the window regained focus (reported on Windows 11). The primary
window's own comment even claimed it was "matching the secondary windows," which
was no longer true.
Hoist the chat-window webPreferences into a single shared factory
(`chatWindowWebPreferences`) in session-windows.cjs and use it for BOTH windows,
so they can never drift on this flag again.
* test(desktop): assert chat windows disable background throttling
Cover chatWindowWebPreferences: it must set backgroundThrottling=false (so the
streaming transcript paints while the window is blurred) and pass the preload
path through while keeping the hardened defaults (contextIsolation, sandbox,
nodeIntegration=false).
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