fix(ci): dedupe the split-pane chat harness page wiring

[CrysisDeu]

Summary

Repo-wide jscpd is red on every merge ref: capture-chatpane-queue-edit.mjs and capture-chatpane-upload-error.mjs carry the same 14-line preparePage stanza (route fallback, websocket silence, split-layout seed, app open). This is the sibling of #6434, which cleared the other clone pair (aws-control / hero-art-proxy) via serveDist.

Extracts the stanza into scripts/lib/prepare-split-chat-page.mjs with a pre hook carrying the upload harness's 400 intercept.

• npx jscpd . in website/: 0 clones (was 1 pair)
• node --check on all three files: clean
• Behavior-preserving: the upload harness reaches the same state before and after (verified by running both versions locally — it progresses to the 2-pane split identically; its banner-wait failure is pre-existing on main in this environment, unrelated to the dedupe)

Unblocks the Frontend Lint & Type Check job for every open frontend PR (first observed on #6419's merge ref).

[github-actions]

UX Review (Fable 5) — ✅ PASS

UX-level review of 788024eeddb28dd906780e5a8168e97ab388df81 — updated in place on each push. A BLOCK verdict blocks PR readiness; PASS/CONCERNS are advisory.

UX-Verdict: PASS

CI-harness refactor only — no user-facing UI, strings, or screenshots change; nothing a product user ever sees.

[UX-REVIEWED] 788024e

[github-actions]

Design Review (Fable 5) — ✅ PASS

Design-level review of 788024eeddb28dd906780e5a8168e97ab388df81 — updated in place on each push. A BLOCK verdict blocks PR readiness; PASS/CONCERNS are advisory.

Design-Verdict: PASS

Textbook extraction of a verbatim clone into the same scripts/lib/ seam #6434 established, with a minimal pre hook covering the one real divergence.

[DESIGN-REVIEWED] 788024e

[github-actions]

GPT 5.6 Review — ✅ no blocking findings

GPT 5.6 completed its review of 788024eeddb28dd906780e5a8168e97ab388df81 and found no blocking issues.

This comment is updated in place on each push.

Review details

No findings.
[GPT-REVIEWED] 788024e

False positive or not applicable? A repository writer can comment:
/ai-review override gpt 788024eeddb28dd906780e5a8168e97ab388df81: <one-sentence reason>

[github-actions]

Opus 4.8 Review — ✅ no blocking findings

Reviewed 788024eeddb28dd906780e5a8168e97ab388df81 — this comment is updated in place on each push.

Review details

No findings.

[OPUS-REVIEWED] 788024e

Verdict parsed from the review's SHA-scoped output markers for commit 788024eeddb28dd906780e5a8168e97ab388df81.

False positive or not applicable? A repository writer can comment:
/ai-review override fable 788024eeddb28dd906780e5a8168e97ab388df81: <one-sentence reason>

[NicholasRBowers]

Tier 1 auto-approve: chore/fix — CI capture-harness dedup (3 files). Criteria: no conflict, no requested changes, security path denylist clean, design-doc gate clean, SAST annotations clean, security checklist all-NO, AI reviewers green. Category: fix, ≤5 files with clear root cause — extracts the duplicated split-pane page-wiring stanza from two offline screenshot-capture scripts into a shared prepare-split-chat-page helper, satisfying the jscpd duplication gate; no runtime surface.