Create unified architecture for memo statistics, filters, and sorting
across all pages (Home, Explore, Archived, Profile) with proper
visibility filtering and consistent data flow.

Key changes:
- Rename HomeLayout → MainLayout to reflect broader usage
- Create useFilteredMemoStats hook for unified stats computation
- Create useMemoFilters/useMemoSorting hooks to eliminate duplication
- Refactor all pages to use unified hooks (~147 lines removed)
- Move Explore route under MainLayout (was sibling before)
- Fix masonry column calculation threshold (1024px → 688px+)

Architecture improvements:
- MainLayout computes filter/stats per route context
- Stats/tags based on same filter as memo list (consistency)
- Proper visibility filtering (PUBLIC/PROTECTED) on Explore
- MemoExplorer/StatisticsView accept stats as required props
- Eliminated optional fallbacks and redundant data fetching

Benefits:
- Single source of truth for stats computation
- Stats remain static (don't change with user filters)
- Reduced code duplication across 4 pages
- Better maintainability and type safety
- Proper security (no private memo leakage on Explore)

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>