#1106 web: follow streaming Thinking and preserve reading position

Description

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 follow threshold before the old check runs.

Capture scroll intent before updating either pane, follow Thinking independently, and preserve manual scroll-up and disclosure choices. Apply the same behavior to the final unpainted chunk; explicit tool-disclosure repaints keep the user's reading position.

Replacement for #1075 after the upstream history rewrite, based on new main `82f46a8c8f475f001ad76d92f58f4a4f8ffb0253`. Contains one fix commit and only `serve/web/app.js` plus its regression tests.

Validation:
- `node --test tools/test_chat_scroll.cjs`: 10/10 pass. Five of the original nine checks fail on unchanged new main; the additional tool-disclosure check was also reproduced failing before its correction.
- `node --check serve/web/app.js` and `git diff --check origin/main`: pass.
- Nine browser interaction checks pass using the actual renderer, scroll functions, click/toggle handlers and CSS with synthetic content in an isolated loopback fixture. No GPU or model inference was exercised.