Add sidebar fixed-item visibility controls

[brsbl]

What was wrong

The fixed top rows had no single ownership surface for visibility and order. Controls were buried in Display options, Automations was outside the same preference model, and hiding every item could leave unnecessary structure behind.

What changed

• adds a top-chrome Customize control with a dedicated Sidebar items menu
• owns New thread, Extensions, and Automations in one ordered, client-local preference
• applies checkbox changes live while the menu remains open
• supports pointer drag reorder and persists it across reload
• collapses the whole top region when all three rows are hidden while keeping Customize available
• keeps Display options focused on organization, sorting, and lifecycle filters, including the Manual label

No server state, wire contract, or daemon protocol changes are introduced by this layer.

Visual evidence

Exact comparison: 4a70ddfa811e1cbc2e9f59be17beb4b1ce84d6f8 → 66cf39b68e813c195e39ecdb022cb5ffbf0292c5.

Before — exact base 4a70ddfa811e1cbc2e9f59be17beb4b1ce84d6f8

The fixed rows have no top-chrome customization entry point.

After — exact head 66cf39b68e813c195e39ecdb022cb5ffbf0292c5

The same root state adds Customize without changing the default rows.

After — menu and live reorder

After — empty top region

How you verified

• Google Chrome for Testing 151.0.7922.71, exact branch web apps from scripts/bb-dev-app, light theme, /, 1440×900.
• Toggled New thread, Extensions, and Automations individually; each updated live and the menu stayed open.
• Hid all three and observed the top region collapse while Customize remained usable.
• Dragged Automations above New thread; both menu and sidebar reordered, and the order persisted after reload.
• Display options showed By project, By machine, Manual, Updated at, Created at, Alphabetical, and lifecycle filters. No page or console errors were recorded.

Fixes: N/A — Updated Sidebar top-region customization layer.

BB-Thread-ID: thr_2e2gbjx943

AGENT GENERATED