Stabilize fresh-agent transcript scroll timing

[danshapiro]

Summary

• move FreshAgentTranscript's auto-scroll-to-bottom DOM write from a passive effect to a layout effect
• add a deterministic regression test that flushes passive effects after an imperative page scroll
• keep the FreshAgentView keyboard scroll contract unchanged

Root Cause

The flaky PageUp/PageDown failures observed scrollTop = 1000, matching the mocked scrollHeight. The only non-user-action writer that can set the transcript scroller to scrollHeight was the passive auto-scroll effect, which could run after the test/user had already performed an imperative page scroll.

Verification

• Negative control: npm run test:vitest -- run test/unit/client/components/fresh-agent/__intentional_missing_file__.test.tsx exited 1 with No test files found
• RED/GREEN: npm run test:vitest -- run test/unit/client/components/fresh-agent/FreshAgentTranscript.test.tsx -t "does not let a deferred initial auto-scroll clobber an imperative page scroll"
• npm run test:vitest -- run test/unit/client/components/fresh-agent/FreshAgentView.test.tsx -t "transcript keyboard scroll"
• 50-run focused FreshAgentView transcript keyboard scroll loop
• npm run test:vitest -- run test/unit/client/components/fresh-agent/FreshAgentView.test.tsx test/unit/client/components/fresh-agent/FreshAgentTranscript.test.tsx
• FRESHELL_TEST_SUMMARY='verify fresh-agent scroll flake stabilization' npm run check
• git diff --check HEAD