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

• Sort 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.
• Validation: 8 focused Vitest tests, TypeScript typecheck, frontend UI audit, and GitHub CI.

Potential risks

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