Add production right-sidebar split panes

[brsbl]

Summary

• add persistent split-pane layouts inside the thread right sidebar while preserving the existing unsplit surface
• support tab tear-out, pane focus, move/swap, maximize/restore, close/recombine, and independent inner-divider resizing
• keep browser, terminal, plugin, file-preview, Info, and Diff tabs bound to their own pane content
• add a production Ladle story and focused layout/persistence coverage

Verification

• pnpm exec vitest run ... — 7 files, 51 tests passed
• pnpm exec turbo run typecheck --filter=@bb/app — passed
• pnpm exec turbo run build --filter=@bb/app — passed
• focused ESLint and git diff --check — passed
• live Electron app at 1280×900 and 980×900 — verified tab tear-out, persisted split layout, inner and outer resize, focus, maximize/restore, close/recombine, and narrow layout

BB-Thread-ID: thr_4rr623umv4

AGENT GENERATED: by GPT-5

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

I am SlopCop, and I am reviewing this pull request now.

This change adds split panes to the production right sidebar. It lets people open and resize several sidebar views at the same time.

I will check security, code quality, architecture, performance, and the main user flow. I will post one final review after these checks finish.

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

This PR lets people divide the right sidebar into several panes. It also saves each thread's pane layout.

I found seven issues. I would fix the first four before merge.

1. The browser deck can unmount before it removes a closed native browser view. A hidden native view can also remain visible.
2. A saved conversation full-screen state loses its only exit control after the sidebar splits.
3. The compact drawer mounts all saved pane bodies, including terminal and browser views.
4. Position-based React keys can move component state between panes.
5. Pane focus calls a parent state update from a React state updater. I reproduced the React warning.
6. A cancelled divider drag keeps changed DOM sizes but does not save them. I reproduced this size mismatch.
7. No-op reconciliation causes repeat synchronous storage writes. Default layouts also create records without a retention limit.

The new code correctly uses the shared split-layout operations. However, it copies the existing split tree and divider renderer.

That copy already lost the existing divider's cancellation restore. A shared divider and stable pane-key helper would prevent this difference.

I found no injection, authorization, or command-execution defect. The native browser lifecycle issue can expose a hidden desktop view and keep private content active.

Validation passed:

• The @bb/app type check passed.
• The 23 focused tests passed.
• The browser test covered split, resize, maximize, reload, and compact layouts.
• The browser test reproduced the pointer cancellation defect and the React warning.
• An independent GPT-5.6 review confirmed the seven findings.

I posted a line comment for each issue. This review uses the comment-only option.