Admin: Section icon selector

[ghostmonk]

Summary

Add an icon picker to the section admin panel so users can choose icons for navigation items instead of relying on slug-based lookup in site.config.json.

Current behavior

Icons are mapped by section slug in frontend/site.config.json under navigation.iconMap. Only 5 hardcoded icons from react-icons/hi are available. Adding a new section requires manually editing the config file.

Desired behavior

• Add icon field to section backend model (SectionBase, SectionCreate, SectionUpdate)
• Add icon picker UI in the admin sections page
• Icon field takes precedence over slug-based lookup
• Support a curated set of react-icons

Files likely involved

• backend/models/section.py - add icon: str | None = None field
• frontend/src/modules/admin/components/SectionsAdmin.tsx - icon picker UI
• frontend/site.config.json - migration path from slug-based to field-based icons
• frontend/src/shared/types/api.ts - add icon to Section type

[ghostmonk]

Implemented across the Editor Command Center phases. Backend icon field on section model with validation, IconPicker component, wired into SectionSettingsForm settings tab.