fix(web): reduce title bar scroll fade height

[maria-rcks]

The title-bar scroll fade was taller than necessary and differed between the normal timeline, settings, pull requests, and other consumers. This obscured different amounts of content depending on the route and viewport.

This change defines a shared --workspace-titlebar-scroll-fade-height token at 1.5rem and uses it for every title-bar scroll fade. The timeline header spacer and load-earlier padding now consume the same token, keeping the mask and content inset aligned globally.

Screenshots

Light mode

Before: 3rem (48px)	After: 1.5rem (24px)

Dark mode

Before: 3rem (48px)	After: 1.5rem (24px)

Made with GPT-5.6-sol in the Codex harness.

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[macroscopeapp]

One finding: the shared fade token silently shrinks the chat timeline's fade band, which was the only consumer relying on the old utility defaults.

^{Posted via Macroscope — UI Consistency}

[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	13.3 KiB	13.3 KiB	+10 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+1 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+9 B (+0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	55.6 KiB	55.6 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	11	11	0 (0.0%)	21	✅
Claude	Total thread wire	13.4 KiB	13.3 KiB	−27 B (−0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	−8 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.4 KiB	−19 B (−0.3%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	56.4 KiB	56.4 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	11	11	0 (0.0%)	21	✅

Baseline: 7880a6e · PR result: 93ae979 · 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: 109.4 KiB
• Claude decoded thread snapshot: 110.1 KiB

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

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR changes the production web UI’s global scroll-fade default, shortening the fade across existing surfaces and synchronizing timeline spacing with it. The change is localized, but the altered product default warrants human review.

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