feat: add tree library view system with grid/list/table modes and sorting

Implements Issue #34 - Tree Library Full View System

Backend Changes:
- Add sort_by parameter to GET /api/v1/trees endpoint
- Support 6 sorting options: usage_count, updated_at, created_at, name, name_desc, version
- Maintain backward compatibility (defaults to usage_count)
- Add comprehensive test for sorting functionality
- All 104 backend tests passing

Frontend Changes:
- Create ViewToggle component for switching between Grid/List/Table views
- Create SortDropdown component for 6 sort options
- Create TreeGridView component (extracted from TreeLibraryPage)
- Create TreeListView component (compact row-based layout)
- Create TreeTableView component (sortable table with columns)
- Update userPreferencesStore with view and sort preferences
- Update TreeFilters type to include sort_by parameter
- Update TreeLibraryPage to integrate new components
- View and sort preferences persist to localStorage

Features:
- Grid view: Best for discovery (default)
- List view: Best for quick scanning
- Table view: Best for sorting and comparison
- Responsive design: Mobile/tablet/desktop optimized
- Table view hides columns responsively
- Sortable table headers with visual indicators
- Smooth transitions and hover effects
- No layout shift when switching views

Testing:
- Backend: 104/104 tests pass
- Frontend: Build successful, no TypeScript errors
- All existing functionality preserved

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/types/tree.ts

@@ -127,6 +127,7 @@ export interface TreeFilters {
   is_active?: boolean
   author_id?: string
   is_public?: boolean
+  sort_by?: 'usage_count' | 'updated_at' | 'created_at' | 'name' | 'name_desc' | 'version'
   skip?: number
   limit?: number
 }