Measure inline chat drop targets within the pane

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The inline chat column lacked a split-drag target marker. Sidebar drags therefore measured the whole main workspace, including the secondary panel, and classified the visible chat center as a left-edge split when that panel was open.

Reproduction report

What changed

Use the existing pane context ID on the inline main-content element. Center drops retain replacement behavior. The change is contained to the secondary-panel UI: 113 added/deleted text lines across 2 files (110 test lines, 3 production lines).

How you verified

• Added the regression test before the production fix. Both clean trusted checkouts reproduced the open-panel failure: expected Replace this chat, received Split left.
• pnpm exec turbo run test --filter=@bb/app -- src/components/secondary-panel/SecondaryPanelLayout src/components/sidebar/useThreadRowSplitDrag.test.tsx src/lib/split-drag src/views/thread-detail/SplitThreadArea: 98 tests passed across 8 files after the fix.
• pnpm exec turbo run typecheck --filter=@bb/app passed; git diff --check passed; no binary changes.
• Headless browser pointer input against real source components reproduced both panel states; the patched open-panel center resolved to replace. Packaged desktop and persisted thread mutations were not tested.
• Public-output model scan clean. The scanner required a Python term-check fallback because system Bash lacks associative arrays.

Fixes #3481

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