#1075 web: follow streaming Thinking and preserve reading position

Descrição

Streaming reasoning can fill the capped Thinking pane while its scrollbar stays at the old position. A large answer delta can also move a chat that was following the bottom outside the 120px threshold before the existing follow check runs.

Capture scroll intent before updating either pane, follow the Thinking scrollbar independently, and preserve manual scroll-up and disclosure choices. The final chunk uses the same behavior when a stream ends before its scheduled animation frame.

Validation: `node --test tools/test_chat_scroll.cjs` (9 tests; 5 fail on upstream base 1735d6471df29b42c26170efaac1f1446a58640f and all 9 pass with the fix), `node --check serve/web/app.js`, and `git diff --cached --check`. The tests model the capped pane and exercise the streaming/final-chunk handlers with synthetic content; no GPU or model is needed.