fix(desktop): stop channel switches jittering and flashing

[Maxwellimus]

Based on perf/switch-tracing — retarget to main after it merges. (Shares the settle seam in ChannelScreen and the community-reset inventory.)

Two visible artifacts made switching feel clunky (diagnosed by frame-by-frame analysis of a 120fps capture):

1. Thread-summary rows reflowed ~3px each a beat (~200ms) after paint — measured as a +6px/+3px/0 differential shift across the viewport in consecutive frames. The summary button was inline-flex, so it participated in its wrapper's line box via its baseline — which moves when the avatar's content resolves (Radix fallback delay, or the avatar image landing after profiles hydrate). The button is now a block-level flex row (wrapper height = padding + fixed button height regardless of avatar state: 36px constant, was 39→36) and the facepile renders its fallback immediately — the participant starts with no image URL, so the 200ms fallback delay only left a blank hole.
2. The timeline skeleton flashed on switches. Two causes: (a) the deferred-snapshot gap — 1–2 frames per switch where the skeleton masked the previous channel's lagging rows — now renders plain background (new "blank" body surface; blocks the same behaviors as the skeleton, only the visual differs; cold loads keep the skeleton); (b) stale revisits — the loading latch reset per switch, so re-entering a channel past the messages staleTime held a skeleton over a fully cached timeline for the whole refetch round-trip. Channels that settled once per session now render cached rows stale-while-revalidate (session registry, reset via resetCommunityState()).

Both are pinned by red-first regression specs: thread-summary-stability.spec.ts (row geometry through delayed profile hydration; failed 39→36px pre-fix) and channel-revisit-no-skeleton.spec.ts (staleness forced by shifting Date.now() past the freshness window; skeleton must never mount over cached rows).

Measured impact (live community, switch traces)

metric	before	after
warm/rapid switch, click→settled paint (median)	260ms	148ms (−43%)
all switches, p90	994ms	682ms
stale-revisit paint	full refetch RTT behind a skeleton	immediate (cached rows)
summary-row reflow on hydration	3px per row, every cold entry	none (spec-pinned)