fix(web): reduce the title bar scroll fade height

[rynfar]

The scroll fade under the title bar was tall enough to wash out the top of the
content behind it, so the first line of a thread or settings panel sat under a
gradient instead of reading cleanly.

Shortens the fade and applies the same height consistently across the chat
timeline, settings layout, and pull requests route.

Adopted from upstream pingdotgg/t3code#8799 (6e324b9bb), cherry-picked clean.

Verified: vp test run --dir apps/web apps/web/src/components/chat/MessagesTimeline.test.tsx
(44 passed), vp run -F @t3tools/web typecheck (exit 0, no errors).

Purely visual, not exercised in a browser — happy to do a real-client pass on request.

Reviewed and integrated with Claude Opus 5 in Claude Code.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.5 KiB	−39 B (−0.3%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+3 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−42 B (−0.6%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.2 KiB	57.1 KiB	−44 B (−0.1%)	66.4 KiB	✅
Codex	Live turn messages	10	9	−1 (−10.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	−3 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	−1 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−2 B (−0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	58.1 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: 9ade80d · PR result: b21bf57 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.5 KiB
• Claude decoded thread snapshot: 110.2 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[rynfar]

Adversarial review found no correctness bugs. It confirmed every topbar-scroll-fade consumer resolves the height from :root with zero references to the old --topbar-scroll-fade-height, that settings and pull-requests are pixel-identical (their removed overrides were already 1.5rem at both breakpoints) so only the chat timeline actually changes, that the timeline is a flex sibling of the topbar rather than overlapped by it — so shrinking the mask cannot push content under chrome — and that no scroll-offset constant (CHAT_LIST_ANCHOR_OFFSET, minimap viewOffset) was coupled to the old header height.

One low-severity note, fixed: the reworded comment on _chat.pull-requests.tsx claimed the top padding is the fade band's height. It is actually WorkspacePageContainer's hardcoded pt-6. Moving the height into :root removed the co-location that made the pairing self-evident, so a future bump of --workspace-titlebar-scroll-fade-height would silently desync the two and wash out controls on both this page and settings while the comment still asserted the invariant. The comment now names the real constraint and where else it has to change.

Re-verified: MessagesTimeline.test.tsx 44 passed, @t3tools/web typecheck clean.