fix(web): clean up composer resize animation

[rynfar]

Reworks the composer context strip's expand/collapse so widths are recomputed on every pass instead of latching remembered values that could go stale, and animates the controls through the Web Animations API at a fixed 180ms.

Cherry-picked from upstream c196f422ed387a1cc2cdb671b0472782e5610339 (pingdotgg/t3code#6209).

Fits Pylon's motion rules: the animation is a discrete, bounded transition rather than a continuous repaint, and it early-returns on prefers-reduced-motion: reduce.

Conflict resolved Pylon-first

BranchToolbar.tsx conflicted structurally, because upstream and Pylon lay the strip out differently:

• Upstream moves the branch selector out of the left group and pushes it right (min-w-0 flex-1 justify-end md:ml-auto md:flex-none).
• Pylon deliberately keeps it in the left run beside the workspace controls (min-w-0 justify-start, adopted as E16 / #5555), because they describe the same thing — where the thread's work happens — while Usage sits opposite.

Pylon's layout was kept and only upstream's animation machinery was taken: the data-composer-context-control markers, the strip's overflow-x-clip overflow-y-visible, and the motion constants. Verified after resolution that upstream's justify-end md:ml-auto did not sneak back in and that ComposerUsageIndicator's ml-auto placement survived. Added a comment at the divergence so the next merge does not silently undo it.

Verification

• BranchToolbar tests pass (56)
• Web typecheck, lint, and format clean across all 4 changed files

Model: Claude Opus 5 (1M context). Harness: 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	11.3 KiB	11.3 KiB	+21 B (+0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	5.5 KiB	5.5 KiB	−1 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	5.8 KiB	5.9 KiB	+22 B (+0.4%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	49.7 KiB	49.7 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	16	16	0 (0.0%)	21	✅
Claude	Total thread wire	11.4 KiB	11.3 KiB	−2 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	5.5 KiB	5.5 KiB	+1 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	5.9 KiB	5.9 KiB	−3 B (−0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	50.6 KiB	50.6 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	16	16	0 (0.0%)	21	✅

Baseline: 0df9a9e · PR result: 7b2adaf · 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: 94.6 KiB
• Claude decoded thread snapshot: 95.4 KiB

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