fix(web-ui): stabilize flow chat follow-output during collapses

[bobleer]

Summary

• Run the same collapse compensation and anchor-lock path while follow-output is active during streaming, instead of letting the RAF follow loop chase scrollTop every frame (which caused visible "sink-down" jitter when tool cards above the viewport auto-collapsed).
• Respect shouldSuspendAutoFollow inside the continuous follow loop so programmatic bottom snaps do not fight collapse protection mid-animation.
• Treat bottom reservation as part of "distance from bottom" for auto-follow, preserve reservations on streaming scrollToLatest (behavior: auto), and add a reactive shrink-clamp restore path that distinguishes browser clamp jumps from genuine user upward scrolling.

Test plan

• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run
• Manual: flow chat at tail while streaming; trigger tool-card collapse above viewport and confirm no conversation-wide sink/jitter
• Manual: wheel/touch scroll up still exits follow-output as before