Sync collapsed sidebar rows and sections through the server

[SawyerHood]

Human comments

What was wrong

After #3304 the structural sidebar settings sync, but the six collapsed-id lists (projects, threads, environments, built-in sections, thread sections, machines) still live only in localStorage.

What changed

Layer 3 of 3 (stack #3306). Base: #3304.

• sidebarCollapsedAtoms.ts: the six collapsed atoms become in-memory synced preferences; their atomWithStorage wrappers and the folder-era storage adapter are deleted. Every existing toggle site already uses functional updates, so conflicting writes from two devices re-apply on top of the server list instead of clobbering it, and toggles made while a request is in flight compose into the next one. RootComposeMobileRecents.test.tsx seeds the atom through a jotai store instead of localStorage.
• Ids of deleted entities are not pruned server-side. The client already drops a deleted project's id on the delete broadcast, a stale id renders nothing, and the registry caps each list at 10,000 entries.

Review

One GPT-6 Astra review round requested changes. All three findings were in the layer 2 sync controller (writes rebased onto the submitted revision, composed functional updates through a conflict, and reconciling the authoritative response after pending writes settle). They are fixed and tested in #3304. A later simplification pass removed this layer's server-side pruning and write debounce as not worth their code.

How you verified

• Sidebar, cache-owner, and mobile-recents suites plus the full @bb/app, @bb/db, and @bb/server public-route suites green; pnpm exec turbo run typecheck --filter='...[origin/main]' green.
• Live in the dev app: clicking a project chevron in one window stores the id on the server and a fresh window opens with the row collapsed; a CLI write containing an unknown id is stored as sent.

AGENT GENERATED

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