feat: Add interactive Argument Flowcharts with React Flow

- Create FlowchartPage.tsx with React Flow visualization
- Custom nodes: thesis (blue), contention (green), attack (red), rebuttal (blue), dropped (amber)
- Stats bar showing node counts by type
- Add/delete nodes with dialog form (label, description, type, strength)
- Connect nodes by dragging (animated arrows)
- Export flowchart as PNG via html-to-image
- Add route /app/flowchart and nav link with GitBranch icon
- Install reactflow and html-to-image dependencies

Author: Programcode