fix(web): reveal scroll-to-end when the composer hides the last message

[rynfar]

The scroll-to-end button stays hidden when the final message slips beneath the composer because the timeline counts the composer's inset as visible message space. Use the actual viewport distance from the bottom, retaining the existing 40-pixel tolerance.

Ports previously approved upstream #9280, split from #269. Applies to the web chat and desktop renderer; mobile uses its own timeline.

Validation: 44 focused MessagesTimeline tests pass, including the inset regression; web typecheck, targeted lint, formatting, and diff checks pass. Integrated browser verification passes at 1440×960 using an isolated environment with disabled providers and synthetic conversation data. At 140 pixels above the bottom, baseline 08ba8b1 hides the button and the candidate shows it; clicking returns to distance 0 and hides the button. Repeated the return path twice. No live provider or production-state changes. The existing LegendList recycling warning is present on both builds.

Before / after at the same scroll position:

Before	After

Video: scroll up, return to the final response, and repeat (7 seconds).

scroll-after.mp4

Implemented with GPT-6 Astra through the Codex harness.

[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	13.6 KiB	−12 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−12 B (−0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	0 B (0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	56.5 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	−2 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+3 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−5 B (−0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.3 KiB	57.3 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: aa12838 · PR result: b14e85d · 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: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

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