feat: add filter controls customer support (#441)

[Timrossid]

Summary

Adds customer support infrastructure to the filter controls, providing inline help popovers and a comprehensive support guide modal for users who need assistance with filtering.

Changes

New Files

• FilterHelpPopover.tsx - Inline popover component for per-filter help (tips + FAQ per section)
• FilterSupportGuide.tsx - Full modal guide with three tabs: Guide (expandable sections), FAQ (all FAQs aggregated), Contact Support (email links)
• useFilterCustomerSupport.ts - Hook containing help content for 7 filter sections (difficulty, duration, price, topics, instructor, content-type, rating) plus state management for popover/modal
• FilterCustomerSupport.test.tsx - 21 unit tests covering popover interactions, guide tabs, hook state management, and accessibility

Modified Files

• FilterSidebar.tsx - Added FilterHelpPopover to all 5 filter section headings, support button, FilterSupportGuide integration
• FacetedFilterSystem.tsx - Same customer support integration as FilterSidebar
• DashboardFilters.tsx - Added Help button in header bar, FilterHelpPopover on Time Range label, FilterSupportGuide integration

Testing

• All 21 new tests pass
• All 11 existing DashboardFilters tests pass (verified no regressions)
• Tests cover: popover toggle/close, keyboard Escape, aria attributes, guide tab switching, FAQ tab, Contact tab, hook state management, help content verification

Closes #441

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