Replaces flat 20-link sidebar with collapsible section groups:
- Multi-item sections (Marketplace, Content, Email, System) are
collapsible with animated chevron; active section always expands
- Single-item sections (Dashboard, Suppliers, Billing, Analytics,
Pipeline) render as direct links — no toggle overhead
- pSEO merged into Content; Users moved into System; new Billing slot
- Unread badge surfaces on Email group header when collapsed
- localStorage persists per-section open/closed state (key: admin_sidebar_v1)
- Mobile: group headers hidden, all items shown in horizontal scroll
(preserves existing mobile behavior exactly)
- section_map Jinja dict derives active_section from existing admin_page
— no route changes needed
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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