fix: stop reasoning auto-scroll from fighting manual scroll-up during streaming

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Describe Your Changes

• The reasoning/"thinking" container's auto-scroll effect in web-app/src/routes/threads/$threadId.tsx ran on every streamed token while status === 'streaming' and unconditionally forced scrollTop = scrollHeight, with no check of the user's current scroll position. Scrolling up to re-read earlier chain-of-thought was reverted on the very next token update, so it was effectively impossible to read anything until generation finished — exactly the scroll behavior described in the issue.
• The fix guards the auto-scroll with a small "near bottom" threshold (REASONING_AUTO_SCROLL_THRESHOLD_PX = 24px), mirroring the stick-to-bottom pattern already used by the main conversation view (use-stick-to-bottom). The box still auto-follows new content while the user hasn't scrolled away, but stops yanking the view down once they scroll up on purpose.
• No new state or event listeners were needed — the effect reads the container's current scroll position on each run, so a manual scroll-up is naturally respected until the user scrolls back near the bottom themselves.

Scope note

Issue #153 bundles several complaints. This PR addresses only the one that was concrete and traceable without logs — the reasoning auto-scroll bug quoted above. The other reported symptoms (app crashes/freezes shortly after generation completes, a "shaky" UI during token generation, and "code rendering is a mess") have no logs, stack traces, or specific repro steps attached to the issue, so their root cause(s) can't be safely pinned from the codebase alone. Those remain open; the reporter would need to attach Settings → General → Data Folder → App Logs plus explicit repro steps (which build/backend, which model, does it always happen, etc.) for someone to investigate further.

Fixes Issues

• Closes bug: Force closes #153 (partially — see scope note above; the crash/freeze, shaky-UI, and code-rendering complaints from the same issue are not addressed by this PR and should be tracked separately once more detail is available)

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features) — logged an ADR entry in AGENTS.md §7
• Created issues for follow-up changes or refactoring needed — n/a, follow-up is tracked on the original issue bug: Force closes #153 pending more info from the reporter

Verification

• yarn workspace @janhq/web-app lint — no new errors/warnings introduced (all pre-existing lint errors are in unrelated files: codeBlockDownload.ts, ttft-timing.ts, thread-read-store.ts)
• yarn tsc -b --noEmit — clean, no type errors