feat: add template flows for common mobile app patterns

[trmquang93]

Summary

• Add 6 pre-built flow templates (Authentication, Onboarding, Settings, Tab Bar App, E-Commerce, Social Feed) that users can insert onto the canvas to eliminate cold-start
• Each template includes SVG wireframe screens, hotspots with configured actions/API endpoints, and connections between screens
• Template browser modal accessible via toolbar button, file menu ("New from Template"), empty state button, or T keyboard shortcut
• Templates are code-split via dynamic import() and IDs are regenerated on insert to prevent conflicts

New files

• src/templates/ -- 6 template data files + wireframe SVG generator + registry index
• src/utils/mergeFlowAtPosition.js -- Centers template at viewport position, delegates to mergeFlow() for ID remapping
• src/hooks/useTemplateInserter.js -- Computes viewport center and inserts template data
• src/components/TemplateBrowserModal.jsx -- Modal with 2-column grid of template cards

Modified files

• Drawd.jsx, ModalsLayer.jsx, CanvasArea.jsx, TopBar.jsx, ToolBar.jsx -- Wire up template state and callbacks
• EmptyState.jsx -- Add interactive "Start from Template" button
• useKeyboardShortcuts.js, ShortcutsPanel.jsx -- Add T shortcut
• userGuide.md -- Document template flows feature

Test plan

• 316 tests pass (24 new: 8 for mergeFlowAtPosition, 16 for template data validation)
• Lint clean, production build succeeds
• Open empty canvas -- "Start from Template" button appears
• Click template button -- modal shows 6 templates in 2-column grid
• Select a template -- screens, connections, hotspots appear centered on canvas
• Insert template on non-empty canvas -- no overlap with existing screens
• Press T -- template browser opens
• Undo after insert -- all template screens removed
• File menu > "New from Template" works