Add sidebar fixed-item visibility controls

[brsbl]

What was wrong

The sidebar’s two fixed top-region rows—New thread and Extensions—had no item-level visibility preferences. The legacy hidden-plugin preference could also still contain the built-in __builtin__/tools key, so introducing an Extensions preference without a one-time, order-independent handoff would either resurface a previously hidden row or consume unrelated plugin-page state.

What changed

• adds client-local visibility preferences for New thread and Extensions, both defaulting on
• adds the decided Sidebar items checkbox group last in Display options and keeps the menu open while toggling
• hides only the selected fixed row, with no overflow disclosure and no route removal
• migrates only the legacy __builtin__/tools entry to Extensions off, preserving plugin-page keys for the plugin-list migration
• keeps migration idempotent and order-independent whether the plugin-list or fixed-item layer runs first
• adds focused coverage for defaults, repeated toggling, persistence, route availability, migration fan-out, and command-palette escape-hatch behavior

No server state, server files, wire contracts, or daemon protocol fields changed.

Before (parent PR #2474 head 23c1937624d5f3a408ba7be3876a461658a03790)

Display options ended at Sidebar sections; New thread and Extensions had no item-level controls.

After (this PR head 7071a5a492b58bf0403792ea8c1e8786c8e41468)

Sidebar items renders last, with both fixed rows checked by default and no new sidebar chrome.

Toggling Extensions removes only that fixed row; New thread and the rest of the sidebar remain unchanged.

How you verified

• focused Vitest acceptance after merging Move thread search into the command palette #2474: 6 files, 51 tests passed
• pnpm exec turbo run typecheck: 76/76 tasks passed
• repo lint: 0 errors (174 warnings)
• Chrome for Testing 151.0.7922.71 against scripts/bb-dev-app current: five toggle cycles, menu retention, hard reload, persisted state, 1440×900 and 1024×768 layouts, light/dark themes, and legacy tools migration; zero runtime exceptions
• Safari 26.5.2 via Safari WebDriver: default and hidden states, hard reload persistence, light/dark/system themes, Extensions client navigation, and return to the composer; zero captured runtime errors
• before/after captures used the same copied fixture database, route, 1440×900 viewport, and state; only the compared revision changed

Fixes: Updated Sidebar Spec — top-region item toggles layer

BB-Thread-ID: thr_2e2gbjx943

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