fix(sidebar): preserve canonical section state

[beruro]

Problem

Sidebar cloud-session ordering and persisted section-collapse state can be overridden by pagination/search projections, causing unstable order or lost user state.

Solution

• Keep the Pinned section from current develop, then sort unpinned cloud-scoped local sessions by canonical activity before pagination.
• Keep persisted collapse state authoritative while search only filters visible rows.
• Add focused ordering and collapse-state regression coverage.
• Keep the ordering invariant compatible with the current Pinned section and backend/local pagination rules.

Potential risks

• Changes to canonical activity timestamps could alter ordering semantics.
• Search expansion must not accidentally persist temporary expanded/collapsed state.

A session missing from sessionById falls back to its incoming order until metadata arrives; this preserves visibility but can temporarily defer canonical activity ordering.

Verification

• ./node_modules/.bin/vitest run for cloudScopedMenuItems.test.ts, menuSelection.test.ts, and workstationSidebarMenuItems.test.ts — 3 files / 20 tests passed on merge commit 1be1125b6.
• pnpm typecheck — passed on merge commit 1be1125b6.
• Focused Prettier and ESLint checks for the three conflict-resolution files — passed.
• git diff --cached --check — passed before the merge commit.
• GitHub CI on the pushed merge commit — PASS: frontend typecheck/lint/tests, Rust Clippy, and AI-attribution history scan.