Structure the sidebar into configurable regions

[brsbl]

What was wrong

The sidebar was one implicit stream: bb-owned Extensions participated in plugin-page ordering through a reserved fake plugin id, top-level regions had no durable structure, and the Custom-mode loose-thread bucket still used the obsolete “Unorganized” name. That made independent section ordering/hiding and stable boundaries impossible.

What changed

• Introduced three explicit top-level regions—New thread / Extensions, Plugin pages, and Thread list—with full sidebar-border seams only between visible, non-empty neighbors.
• Added client-local, normalized order and visibility preferences plus a Sidebar sections group in Display options. Thread list is permanently unhideable; the other two regions hide and restore independently.
• Moved Extensions beside New thread and removed its reserved plugin-order key and plugin-row menu behavior.
• Renamed the desktop Custom-mode loose-thread header and delete-section dialog copy to Threads.
• Added focused persistence, recovery, keyboard-order, divider, Extensions, and compatibility regressions. No server, daemon, wire, or plugin-SDK contract changes.

Screenshots

Before — merge base 42d2c1bec

After — PR head c781d323e

How you verified

• pnpm exec turbo run typecheck --force — 76/76 tasks passed on Node 22.22.1.
• pnpm exec turbo run lint --force — passed with zero errors (181 existing React compiler warnings).
• Focused Phase 1 suite — 7 files, 37 tests passed.
• @bb/app full suite — 434 files, 3,362 passed / 3 skipped.
• @bb/desktop — 35 files, 231 tests passed.
• Repo-wide forced tests ran across 74 tasks excluding the independently reproduced base-failing @bb/provider-parity; 70 passed. Load-sensitive server and integration failures passed in isolation. Remaining unrelated baseline defects were the macOS /tmp vs /private/tmp assertion in @bb/plugin-build and the recorded Claude fork replay producing no events.
• Google Chrome for Testing 151: default three-region/two-seam layout, each hide state, persistence, reorder, Display options, light/dark, and 390 / 768 / 1440 / 3440 viewports; no runtime exceptions.
• Safari 26.5.2: settled load, hard reload, Display options, Thread-list no-hide contract, reorder, and persisted reload all passed.

Fixes # — no linked issue.

BB-Thread-ID: thr_2e2gbjx943

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