Scope the split workspace layout to one tab (#873)

[SawyerHood]

Fixes #873.

Cause

The main content area no longer renders "the thread in the URL" — it renders the split layout, which lived in a localStorage-backed Jotai atom (bb.splitLayout) whose storage subscribe hooks the window storage event. That event only fires in other tabs, so the moment tab 1 selected a thread, every other tab's atom was overwritten with tab 1's pane and repainted.

Each tab's sidebar highlight and address bar come from useRouteState() (react-router, per-tab) and never moved — hence the exact split-brain in the report: main pane = tab 1's thread, sidebar + URL = tab 2's thread. bb.splitLayout.maximizedPaneId had identical wiring.

Fix

New createTabScopedStorage() in browser-storage.ts:

• reads prefer sessionStorage (per tab, survives that tab's reload), falling back to localStorage so a newly opened tab still starts from the most recent arrangement;
• writes go to both;
• no storage subscription, so one tab never adopts another tab's value mid-session.

splitLayoutAtom and maximizedPaneIdAtom use it. Genuine user preferences (theme, sidebar collapse, model defaults) keep syncing across tabs as before.

Tests

• SplitThreadArea.test.tsx — "ignores a layout written by another tab (issue Selected thread persists across tabs #873)": simulates the other tab's localStorage write + storage event and asserts the pane and URL don't move.
• atoms.test.ts — layout and maximized pane unaffected by another tab's write; new-tab seeding vs. own-reload restore.
• All four new tests were confirmed to fail against the unfixed atoms.ts and pass with it.
• ThreadRow.test.tsx's reset now clears sessionStorage too (it was leaking layouts between tests once writes hit both stores).
• turbo lint typecheck test --filter=@bb/app: 0 errors, 283 files / 2083 tests pass.

End-to-end verification

Ran the dev stack with three real threads and drove two real Chrome tabs.

Before (fix reverted, same harness) — the reported bug:

AFTER tabA: /threads/thr_C  pane "Tab C"
AFTER tabB: /threads/thr_B  pane "Tab C"  sidebar [thr_B]   <- bleed

After:

1 repro  tabA: /threads/thr_C "Tab C" | tabB: /threads/thr_B "Tab B" sidebar [thr_B]
2 reload tabB: restores its own thread, not tab A's
3 split  tabA: 2 panes [thr_C, thr_A] | tabB: still single-pane "Tab B"
4 newtab      : still seeds from the last arrangement

Note

A brand-new tab still inherits the last-written arrangement (case 4) — pre-existing "restore your split on open" behavior, and now the only cross-tab path left. If new tabs should instead start from just their URL, dropping the localStorage fallback in createTabScopedStorage is a two-line change.

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