Fix single-pane split-drag drop zones and default a center drop to split

[elb-comt]

What was wrong

In a single-pane workspace, dragging a thread from the sidebar onto the chat pane splits or replaces depending on whether the right panel is open. Root cause: the single-pane chat region has no data-split-pane-id, so beginSplitDrag falls back to the whole <main> for its drop rect. In the inline single-pane layout <main> also contains the right secondary panel (a sibling react-resizable-panels panel), so the zones are measured over chat + panel. With the panel open the visible chat is the left half of <main>, so the visual center lands in the left band (25% < 28%) and splits; with the panel closed the visual center is 50% and replaces.

Repro, root cause, and code permalinks: #3481.

What changed

• apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx: tag the inline pane's chat region with data-split-pane-id so drops are measured against the pane that owns them. The tagged element is the chat column, which excludes the right panel. Behavior no longer depends on the right panel.
• apps/app/src/lib/split-drag/zones.ts and the two sidebar drag hooks (useThreadRowSplitDrag, usePaneContentSplitDrag): when the workspace has exactly one pane, a center drop resolves to a split to the right instead of replace. Replace stays available via an edge drop, the pane cap, and clicking the thread.
• apps/app/src/lib/split-drag/zones.test.ts: unit test for the single-pane center case.

No wire, CLI, or guide changes.

How you verified

• Unit: new zones.test.ts case "splits a center zone when the workspace has a single pane"; existing decideThreadDrop cases are unchanged because singlePane defaults to false.
• Manual against the running 0.42.1 UI (local server, Playwright): injecting the equivalent data-split-pane-id on the inline chat region and re-running a real sidebar drag gives panel-open center → Replace this chat, panel-open left → Split left, panel-closed center → Replace, panel-closed right → Split right. Before the fix, panel-open center → Split left.
• I could not run the monorepo test suite locally (no node_modules installed here); CI should run the @bb/app typecheck/test filters.

Fixes #3481

AGENT GENERATED

[github-actions]

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