PR 5b: Browser chrome — Properties↔Cart rail, delete-409 dialog, Appears in N Views

[allison-truhlar]

PR 5b — Browser chrome for Neuroglancer Views

Stacked on #426 (ngviews-05a-cart-pipeline). Read-only scope. Adds the Browse-side chrome for the Views feature — no backend changes (the 409 delete guard and the /views endpoint already shipped in PR 2).

Built via subagent-driven-development from docs/superpowers/plans/2026-08-07-ngviews-05b-browser-chrome.md — 7 tasks, each with a spec+quality review gate, plus a whole-branch final review and one polish fix wave. Full frontend suite 360 passing; node-check at the stack's known 5-error baseline (pre-existing, untouched files).

Part 1 — Right-edge Properties↔Cart rail + cart drawer

• useLayoutPrefs gains an ephemeral propertiesDrawerMode: 'properties' | 'cart' + selectDrawerMode — reuses the existing "properties" panel id, so layoutConstants/layout-persistence keys are untouched (no new panel-combination keys).
• New BrowseRightRail (Browse-only, always-visible right strip): Properties ⓘ + Cart 🛒 with a live count badge (useCartCount), aria-pressed on each.
• PropertiesDrawer renders a cart body (<CartList/> + "Open full Layer Cart" → /ngviews?tab=cart) when in cart mode, outside the file-selection gate.
• <CartList/> extracted from NGViews so the /ngviews tab and the drawer share one implementation; NGViews reads ?tab=cart.

Part 2 — Data Link delete: dependent-Views confirmation

• useDeleteProxiedPathMutation now preserves the backend 409 {detail:{message,dependent_views}} body via a manual status branch (not sendRequestAndThrowForNotOk, which drops it), throwing a typed DependentViewsError; adds a confirm? payload → ?confirm=true.
• handleDeleteDataLink(proxiedPath, confirm?) re-throws the typed error; the shared DataLinkDialog delete branch shows a confirm sub-view listing the caller's own dependent Views and re-issues the delete with confirm=true. Fixed once in the shared path → covers both delete entry points (Properties drawer + Data Links table).

Part 3 — Properties "Appears in N Views"

• New useViewsForDataLinkQuery hook (mirrors fetchViews; GET /api/proxied-path/{sharing_key}/views, 404 → []).
• New AppearsInViews section in the Properties Overview tab, shown only when the selected file has a Data Link.

Deferred (tracked)

• "Dot on ⓘ" when a file is selected behind the open cart (design §7) — needs cross-panel selection wiring.
• Making the "Appears in N Views" names navigable — waits for PR 6's embedded viewer.

Stack

01→02→03→04(#425)→05a(#426)→**05b**→06(embedded-readonly).