feat: add comprehensive connection drawing system

Implemented full connection drawing and configuration functionality:

Visual Components:
- SVG overlay layer with arrow markers for directional connections
- Pulsing blue glow on source nodes during connection mode
- Cyan highlight on hover during target selection
- Connection mode indicator with slide-down animation
- Curved Bézier paths with smart routing

Interaction:
- Ctrl/Cmd + Click node to start connection drawing
- Click target node to complete connection (prevents self-connections)
- Escape key cancels connection mode
- Click connection line to show properties panel
- Delete/Backspace keys remove selected connection
- Real-time line updates when dragging connected nodes

Connection Properties Panel:
- Protocol selection: HTTP, HTTPS, TCP, UDP, gRPC, WebSocket
- Direction toggle: unidirectional (→) or bidirectional (↔)
- Encryption toggle: TLS/SSL enabled or plain text
- Port mapping: source port → target port configuration
- Delete connection button with confirmation

Template Integration:
- All 6 templates now include pre-defined connections
- Three-Tier: Frontend → Backend → Database
- Microservices: Gateway → Services → Shared DB
- Event-Driven: Producers → Queue → Consumers
- Full-Stack: Complete multi-tier topology
- Connections auto-render 300ms after template load

Connection Data Structure:
- Stores: from/to node IDs, protocol, direction, ports, encryption
- Validates: no duplicate connections, no self-connections
- Updates: node positions update connection paths in real-time

Tutorial Integration:
- Added Link Configuration tutorial (10 steps)
- Teaches: data flow, directionality, protocols, network security
- Added tutorial card to selection menu

UI Enhancements:
- Template Library modal with 6 pre-built stacks
- Image Comparison modal for base image selection
- Load Template button on dashboard

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

Author: Jonathan D.A. Jewell