Add production right-sidebar split panes

[brsbl]

Summary

• add persistent split-pane layouts inside the thread right sidebar while preserving the existing unsplit surface
• render split tab groups side by side in the existing single sidebar header; pane content does not duplicate header chrome and split resize seams are transparent at rest
• separate adjacent split-tab slots with the sanctioned border-seam-vertical/60 token while leaving the stronger page/sidebar divider unchanged
• project the persisted split tree into the shared header so header slots and pane bodies resize together continuously, whether the drag begins in the header or pane divider
• make every shared-header separator drag-resizable, including stacked split positions, and commit the true pointer release coordinate without turning a click into a resize
• reuse the existing split-position menu and commands for active-tab positioning, pane resizing, full-screen/restore, and drag-to-recombine
• preserve compact behavior by rendering one active drawer surface below 768px while retaining the saved wide split layout
• remove the closed queued-label parent layer so this PR is standalone against main

Review feedback

• resolved the existing review threads: compact drawer mounting, stable pane keys, persistence retention/no-op writes, native browser lifecycle, pointer-cancel restoration, full-screen exit access, and updater side effects
• resolved final review findings for stacked header resize, release-coordinate persistence, captured-pointer isolation, and no-op separator clicks
• added focused regression coverage for shared-header geometry, subtle tab separators, horizontal and stacked resize synchronization, release and cancellation behavior, and no-op persistence

Verification

• pnpm exec vitest run ... — 3 focused files, 35 tests passed
• pnpm exec turbo run typecheck --filter=@bb/app — passed
• targeted ESLint and git diff --check — passed
• real BB interface verified at 390×844, 768×900, 1440×900, and 3440×1440
• visually verified one unchanged header, adjacent split tab groups, hover options, left/right/top/bottom positioning, active-tab targeting, inner and outer resize, full-screen/restore, recombination, compact drawer restoration, eight sidebar panes, and eight workspace panes
• visually verified synchronized tab/pane edges at initial, active mid-drag, 27/73 narrow/wide, 72/28 wide/narrow, repeated resize cycles, and a stacked header drag with synchronized header-width/body-height allocation

BB-Thread-ID: thr_4rr623umv4

AGENT GENERATED: by GPT-5.6 Sol

[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.