perf(desktop): prefetch the channel window on sidebar hover intent

[Maxwellimus]

Based on perf/switch-tracing — retarget to main after it merges. (Shares the message-window queryFn hunk with the tracing instrumentation.)

A deliberate hover on a sidebar channel row (100ms dwell — pointer scrubbing never fires) warms that channel's message window, so the subsequent click paints from cache instead of paying the relay fetch on the switch path. Live traces measured that fetch at 180–310ms per cold channel entry on a real community — this moves it into the hover dwell, ahead of the click.

The window's query options move into a shared factory so the prefetched entry is byte-identical to what the mounted query reads; the prefetch respects the window's staleTime (hovering a fresh channel never refetches) and dedupes with in-flight fetches. Forum channels are excluded, matching the window query's own gating.

The mock bridge gains a head-fetch probe (cursorless window reads) and a regression spec asserts both directions: dwell fetches exactly once before any click; scrubbing across rows fetches nothing.